They are not respecting you and your husaband's position as parents. I don't know if that makes it toxic. They could have gone about this whole thing in a more respectful way and still covered these topics.

With the bat, it sounds like in-laws are trying to overcompensate for DS15 having a messed up childhood due to an absent BM so they cater and coddle him over the other kids. He is the "problem that needs fixing" and they want a project. 

It sounds like your in-laws have poor boundaries. They need to understand that you are not a stepmom, since you legally adopted SS15 and since you are legally his parent, their opinions hold no weight. You and DH have total say, period. "No" is a complete sentence. You do not need to tolerate disrespect and if they want to continue, they can lose tueir place in your life, DD3's life, this baby's life, and possibly DH and DS15. Sounds like your in-laws haven't changed since the last time so maybe a more permanent break is needed with no apologies and make-ups this time. 

Then inform MIL and SIL that YOUR son will now have zero interface with them as they are invasive, toxic, manipulative and will no longer be tolerated as the invasive Harpys that they are while smacking them in the face with the adoption papers.

You are SS's mom and MIL and SIL need that fact crammed up their idiot asses wholesale for the next 3 years.

If SS makes it to 18 without a record dump him at the military recruiters office on his B-day, or HS graduation whichever is the later. See if the move in with Aunty offer stands when he becomes an adult.  Betcha the offer will have been long forgotten by then.

Time to go full control parent on SS as well as cutting the toxic Harpy squad out of his life and the lives of the rest of your kids and out of your marriage.

This is not something to discuss with your DH, this is something you tell your DH and make sure he understands that he has your back in this and keeps his idiot mother and sister in their place or he and your 15yo can both go live with them while he pays a shit ton in $CS for your LOs.

Grrrrrr!

Stop being the whipping post of the mult generational family idiocy in your DH's  clan's polluted gene pool.

Save yourself and save your own kids from that cesspool of idiot genetic effluent.

Grrrrr. Again.

I think they might have wanted to get a reaction out of you and they did. I try to pick my battles, so I might let some things go that others wouldn't, but if these two don't visit often, I personally would not have "sweated" the gift. I also will dig my heels in until they bleed if I can sense someone is setting me up for conflict or if they seem to be pushing for a reaction.  I'm stubborn that way. I don't think any of you are being toxic. I think they crossed boundaries and might enjoy a little drama, but if you all don't see them often, I might tolerate a little bs once a year or so. But that's me. Take care of yourself! 

They sound kind of messy and meddling. 
 

Old school family / mother in laws / grandparents think they know everything sometimes. And sometimes you have to check them like "look Betty. I appreciate your opinion however you had your turn to raise your kids how you saw fit, these are my kids and I will raise them how I see fit. If I said no gifts over x amount/gift ALL my kids equally or we cannot accept ... then that's what it is!" 
 

And your husband should be enforcing boundaries with his family and/or backing you up .... IF he knows what's good for him and wants to keep his marriage in tact. 
 

Who argues with a pregnant person?!? If I were in this position I would have kicked them out the moment they fixed their crusty lips to argue with me while I'm pregnant causing unnecessary stress on an unborn baby should be child abuse. 

So yeah I do think it's toxic behavior to argue with people who are more medically delicate such as pregnant women. 

I think they are still being difficult because they had such a large part in raising your son before you came along. They have never come to terms with the fact that you are now his Mother. They haven't shifted back to being grandparents and aunt, they are still feeling like parents. Honestly, I think you need to limit your son's contact with them and he certainly shouldn't be alone with any of them. If they are talking about him moving in with the aunt in front of you, imagine what they are saying when you aren't around. And your DH needs to really step up and take over the communication with his family. He needs to tell them once that his son is not moving in with anyone and then refuse to discuss it any more. He needs to take the heat from them, not you. The fact that this is till an issue after 5 years is just ridiculous.

It might be worth having some sort of reset with your son. A sit down with you, your husband and him where you tell him that you know that things have been difficult at home and there has been more tension, but that is normal in any family especially during stressful times. It is not an excuse to runaway to greener pastures and avoid dealing with the consequences of his actions.

Let him know that he will be staying in the family home until he is 18 and that you love him. Tell him that everything you are doing is because you believe that it is the best for him to learn from his mistakes now, rather than continue to make the same mistakes as he gets older.

Then I might try to get some sort of by-in from him and ask if there are any reasonable changes that could be made that might make things better for him in your home. Asking him doesn't mean that you have to agree to any of them but perhaps he will come up with something reasonable and it will show him that you want to listen to him. 

You adopted that boy.. he is now YOUR child.  If you have to go to therapy with him to work through issues with his anger etc.. then you need to do that.

Toxic relatives or not, you don't send your child away.. you raise your child.  

Your MIL and SIL sound meddlesome.  I recall the history with them.. they felt a lot of connection and "ownership" over your DS15 because from initial posts.. it seems that the boy was raised in their home in his early years and they heavily helped/overfunctioned for your DH prior to you coming into the picture.

You have always been at odds with them.. resentful of their interference.. and probably in many ways rightfully so, but as  you also say.. you often lose your temper in outsized ways.. and this is not new or only a hormonal issue.. and maybe you could also try to work through better response mechanisms and conflict resolution with an individual counselor of your own

In the end, I don't think their intentions are actually bad... I think they are susceptible to the sob story your DS might paint.. and their history with you is definitely going to impact their relationship with your other bio.. because you are a gatekeeper there..where with the boy.. they don't need a gate.

He should , must have a talk with his mother and sister.  And tell them off. To eatger stop all of this or not to see DS.