When it comes to possible pinching/pushing the little ones, I think you need to express your fear to your DH. That he will have to be extra observant this visit. Not only the first half sibling (totting around much more than last visit et)but also because here comes another one (new baby) .... as little time as SS spends in your home he may be jealous, act out over yet another new sibling and/or simply play too rough.

Half sibling or not, it's really not much difference in this long distance occasional visiting SS than if he were a friends child .... you obviously have to supervise and be cautious. Accidents happen, and yes, sometimes some actions are deliberate. 

He has to be aware and stay on top of all the interactions. In fact he may have to be more alert to surroundings/happening with a new baby coming anyway. Not just due to SS, but your own toddler may have some adjustment periods with a new baby arriving (toddler won't be the family only child in home anymore and/or the sole focus of kid attention). There could be an incident of jealousy or simply learning to play very carefully between your own two. 

As to influence, the best part of that is you and Dad have at least 80% of daily year round influence of your toddler and will too with the baby. While, yes, toddler may see/hear some negative undesirable from the oldest child on SS's visits, you simply have to have no tolerance of it affecting your own child. Example, Dad must discipline his oldest son when unacceptable behavior occurs and he must discipline the toddler if toddler tries to repeat the behavior. Actually just like if all the kids lived in your home full time. You wouldn't let the oldest say/do things against the house rules, so why let him try it on his occasional visits? Consistency. 

I brought a baby into our blended situation.  The SSs didn't care much andBD9 doesn't know them.  BD9 only knows one of her half sisters on DH side.  After OSD and YSS attacked DH, I cut off all contact with them and BD.  She was still little then so it was easy.  

I will say that any relationship your little ones have with their older siblings is on DH.  He will need to set the tone for SS and his expectations.  He will need set SS straight if BM fills him with programming to cause havoc with the little ones.   Teach and expect him to be the big brother.  Come down hard on any abusive behaviors.  Don't let any special snowflake, whoa is me  who is being replaced BS take root.  Its not true for DH and he should make it clear that his heart is big enough for all his kids.  

Your job is make sure DH stays clear on what his role is in making family happen in your home.  Don't let BM slip in.  IMO

It’s interesting, this made me think of my childhood, from a different perspective: I had a SM, and her and my dad had my half-brother together when I was about 8. My dad had full-custody of me, and I remember wanting to be around the baby (no resentment until my SM created it between us), but my SM would NOT let me have anything to do with him. As he got older, she would flat out tell him to stay away from me, right in front of me. She also hated me and was verbally abusive to me. My brother was also treated 100% differently than me. He had no chores (I was timed on my chores and not completing them in the time limit meant I was grounded to my room for the night), he had his very own candy drawer, went on day trips that I didn’t get to go on, got any toys his heart desired...the list goes on. Anyway, him and I had NO relationship whatsoever until he turned 19, when we were around each other during Christmas and realized we actually had a lot in common. All of that time wasted by my SM alienating him from me. But, my SM truly is an evil person. 

Obviously your situation isn’t exactly the same, and you have seen some behavior that’s concerning. I guess I’m just saying, I’ve been on the other side of this equation, in your SS’s shoes, and personally I just wanted to feel loved and included. Just be cautious to gauge the situation as accurately as possible. I think you should encourage a relationship, but make sure his interactions with DS and the baby are supervised until you feel comfortable that you can trust him. If he shows aggressive behavior that is truly concerning, that is another story entirely. 

My brother and I are very close now, and it makes me sad that we missed out on so much time not having that relationship growing up.

Congrats! I'm also in my third trimester! I have a 3 year old and I have two skids, SS14 and SD18. SD18 is great with DS3. She babysits him for hours at a time, is very loving, and is less than 2 hours away at college. If I need an emergency babysitter when I go into labor (before my parents can drive here) she will be my go to and I feel comfortable with it.

SS14 is a completely different story. We've had both of the skids either full-time or half-time. We currently have SS14 half of the time, but he has little to no relationship with DS3. When I first moved in together when DH, SS was about 8 or 9, he started pushing and kicking my dog because my dog just wanted to be around him and greet him enthusiastically (my dog is 15 pounds). He has also done the same thing to his grandma's puppy when visiting her. He regularly expressed jealousy before DS3 was born, but this time around, he just says that he has no feelings about the baby. He has never been left alone with DS under my watch. DH is more lax about it and I call him on it if I see that he's allowed them to be alone together. There are rare occasions though when we are all out together that he shows caring for his little brother, like leading him away from a street, so I don't think he's a completely terrible person, but he is one with lots of issues.

What annoys me is that DH tries to push SS to have a relationship with DS. My DH is also oblivious of what goes on around him and in denial about his teenage son, so I have fears what SS will do. Honestly, it's one of the reasons why I won't divorce DH until my kids can verbalize what is happening to them.

DS, on the other hand, loves both his siblings. He's only 3 so he has no idea and I don't think he or the new baby will ever have any of the ill feelings that I have towards SS. SS will be out of the house in 4 years. DS's entire understanding of sibling relationships of that type are what he sees with his own. It's normal to him that SD plays and babysits him and SS ignores him.

In terms of the importance of half-sibling relationships (and full siblings too), I think it depends on distance, parents, and personality. DH has both full and half siblings. I think his relationship with all of his siblings birthed by his mom are really pretty equal in terms of how much he contacts them. He left home young and never moved back to the area, so although he contacts them a lot through social media, he only sees them once or twice a year because his mom hosts us during those times and she connects everyone by hosting holiday meals and such. On the hand, his other siblings (even the half ones) are most likely much closer emotionally due to the fact that they all still live in the same small city. DH has the most in common with one of his half-siblings and I also see this and I like this half-sibling the most too. And, he and this half-sibling are about the same ages apart as SD18 and DS3. DH's younger full-sibling brother had jealousy issues, ADHD, and behavioral issues as a child (similar to SS) and I think if it wasn't for DH's mom keeping them all together and him living close by, he and his half-siblings might not be so close.

Add to that, DH has a half-sibling that he has no contact with. It is the daughter of his father (his father died when he was young). DH's mom's relationship with her stepdaughter was fraught and after DH's father died, she stopped contact with her stepdaughter, so none of her kids had contact with her during their childhood after their father's passing. She's never been invited to DH's mom's events. DH's mom will even on occasion, think back to how horrible her stepdaughter was, and will be filled with emotion. She has even told me that being a stepmother was the worst. I think she understands my pain with SS and she makes it a point to take the skids a week or two every summer and I sometimes think she does it for my benefit too.

For us, we have a rule in place that SD, age 8, is not allowed behind closed doors (bedrooms) with our children, ages 2 and 11 months, unless being supervised by myself or SO.  We had to put this in place due to SD consistently lying, accusing the 2 year old of hurting her. I also walked in to her aggressively jerking a toy away from the 2 year old. After talking with her, SD admitted to bullying and lying. As far as I’m concerned, this rule will stay in place until there are some major behavioral changes in her and I am comfortable with her being unsupervised (which will likely be never). 

I, too, am worried that her behaviors will have an affect on our children. Honestly, I try to keep them separate as much as possible. I keep trying to teach our children right and wrong, manners, and the proper way to behave. They have consequences  and rules. 

It is very hard living in a blended family where one child is raised completely different. 

When my future SD met my bio twins for the first time I could tell she was giong to have a problem sharing the "spotlight". She was 5 at the time and my boys were 3. We all moved in together when SD was 7 and the boys were stb5. I will NEVER forget the time we were visiting my family and she did not know I was walking directly behind her while she was directly behind my boys. She had an open, full, bottle of water in her hand and for some reason (nothing was even said) she lifted that bottle and tried to smash my son in the head with it. I grabbed it inches from his head, spilling water all over her and him. Of course daddy didn't believe his precious princess would do such a thing and I misinterepred what I thought I saw.

Less than a year later SD thought nobody was watching (DH and I - not married yet were able to see her) and she pushed my other son down when they were walking to their bedroom (we were visiting family out of state). She had just turned 8 and the boys were about to turn 6. I got up and pushed her down on the pillows behind her. (I realize NOW this was probably not the best answer, but I was upset and she was old enough to know better). However, she never laid another finger on them or tried for that matter.

Maybe the best answer is do to them what they do to others. If your SS pushes, pinches, punches your child, do the same in return (just not as hard). This works on toddlers when they are in their biting stage; maybe it'll work on older children too.