The thought of not knowing anyone otherthan your Dh and your MIL and your FIL (which obviously you do know) , wouldn't bother me in this situation. It's not really a social event were you'd be expected to mingle and introduce yourself while making small talk.

IMO, I'd be more comfortable doing the visitation rather than the funeral, if there is a visitation. For one, it's shorter duration and an exit can be fairly in short duration. You'd be going to support your DH and with the exception of dressing funeral home appropriate and quietly and politely expressing sympathy not much else is really required of you. 

While, yes, the visitation/funeral is for BM's parents, and she may plant herself next to the casket for the entirety , it's just a few moments you'd have to be in her vicinity. Unless you feel BM would react poorly (make a scene, for example) because of your presence , however harmless on your part, I'd not let the thought of BM worry me too much. I can be civil and polite , following the 'rules' of funeral etiquette for short durations. 

I would have thought BM would not of issued in invitation, as open visitations and funerals are not usually by invitation but more so to people having a genuine desire to pay their respects to the deceased person. In some cases the people attending a visitation have never even met or known the deceased, but attend to show respect and offer sympathies to the survivors of the deceased (in your case, BM and your SD). If your DH had a decent relationship with the deceased and spent years as a part of the deceased's family, I find nothing wrong in attending the event to show his respects. 

If BM is going to act the drama queen and/or react very poorly to his presence, then he needs to stay away. 

The four of you (DH, you and your in-laws) could choose to do the visitation, pick your in-laws, do a short stop at visitation to show respect and be on your ways having dinner together afterwards. The actual visitation doesn't have to really anything more than if you'd be stopping by a co-worker's deceased parent's visitation where you felt ignoring it completely wasn't something you wanted to do, but over staying and/or making a 'big thing' about your presence isn't your goal either. 

I think I'd tell SD (if I were her father) that I'd worry about it when the time comes and offer her now a bit of verbal support that I know how hard it is to watch a loved one begin to fade (I'm sure your Dh can think of something to express to his daughter he cares about her feeling while she goes through her difficult time). 

This is a topic that causes me anxiety on multiple step fronts.

Regarding your specific question, I would go if DH went. However, BM only has GBM left, and DH would spit on that woman if she were on fire. I highly doubt he would go, and the only way he would go is if I were available and went, too.

When MIL and FIL pass, BM will more than likely be there, and DH will have to run damage control with his family. FIL's family will be cordial but cold. MIL's family will cause a scene. I'll throw them all out if they make life difficult for DH in any way, shape, or form.

I also worry about this issue with my mom and SF. When my mom dies, my dad will come, but I think my SF will handle it with grace. My dad knows boundaries(ish), so I think he'll say his condolences and check on us kids. My SF's funeral, however, God help his XW and my SSis if they show up. His (and potentially my mom's) will be a redneck showdown. I WILL have to bounce people from the viewing. I WILL have to forbid people from coming to the funeral.

So, basically, if you can be cordial and you have been asked to attend, go with your DH while he comforts his kids. Don't stay too long, and sit in the back. If BM walks by, offer your condolences and move along. If she gets a bug up her rear, grab DH and go. Come up with a game plan beforehand and stick to it. The kids Will appreciate his presence, but they will also be thoroughly distracted by other family. You won't need to spend very long there because your DH will likely be pushed out of the way in short order.

Yes- Were I in your shoes, I would totally go, emotional support for DH only. I would also want to be there with DH in case BM tried something sneeky, or draw him in with hidden motives....Won't matter if you don't know anyone else, go pay your respects and leave....

In our funeral scenario - HCBM only attended so she could march skids through like MOTY. (her weekend so we were left with no choice)

But it started a discussion among SO and myself. He started he wouldn't stop her from attending his family funerals in the future out of respect for skids. BUT we both emphatically requested she NEVER attend ours! I'm like look if something happens to me you better stop that crazy bi*ch at the door or I will haunt you forever - I deserve one boundary even if it's in death!!

My personal thoughts have been that it really may only be necessary when children are minors.. that they need their parent's support at a funeral.  But, if someone had a long standing relationship with someone in their EXes family and the EX is not opposed to them coming, I see that as acceptable for him to go pay his respects.  I mean, in the end, it can be possible to put acrimony aside and support people we have known for years that may have been related to but are not the EX.

Then again, I didn't even tell my EX that my mom passed away.  WE didn't have kids together so we aren't in contact (so nice..lol).

Would I ever be in this position though with my DH attending someone in the Exes family?  NOPE.. haha  my husband hates funerals so much he is liable to miss his own.  Now, I do somewhat worry about the MOTY GUBM trying to attend his parent's funeral when that time comes.. If she does... I will just civilly ignore her.  I would owe it to the family to  not cause drama... and not be a party to it.

I do kind of like the attending visitation perhaps when the EX wouldn't be there.. that might be a good compromise.  As far as whether I would go to an EXes family funeral.. probably not but I wouldn't stop my DH from going if he wanted to.

I've been worrying about these scenarios lately because of my histrionic SD and her even more histrionic BM.

Notsobad, why can't SD's mom support her? BM and SD will share in their grief, which is more meaningful support than your husband showing up (when he doesn't really want to be there) and then dragging you along (when you don't want to be there) with no meaningful personal connection to the dead.

Is it surprising to you that SD says BM wants your husband there? If it is surprising, that's probably SD talking and NOT her mother. I'd think long and hard about the likelihood of BM wanting her ex and his wife there. Sounds like SD manipulating to get her dad there.

The risk of an ex (or step) attending is that people who are legitimately there to grieve may be distracted from honoring the life of their loved one if they're tense or worried about drama exes or steps might cause.

The other thing that comes to mind is the crazy ex situation. I am certain my husband's ex will use the opportunity of his mom's death to show up and make everyone uncomfortable and tense even though their acrimonious split was decades ago, my MIL hated her and they live on opposite sides of the country. That's just who his ex is. That will probably ramp up SD, who is already a drama queen on a good day. On top of my husband's grief of losing his mother, he will fear what his ex-wife might pull and how his daughter will react. That's just not right. But I am sure SD will want her mother there - just seems to be how these people operate: with little regard for what's appropriate or kind.

I'm not sure when funerals became spectator sport. It seems to me, if you don't have an emotional relationship with the dead person or someone very close to the dead person, you shouldn't be there. I know the "there to support so and so" thing, but my girlfriend has been attending funerals of her co-workers' parents' brothers-in-law's ex-wives and I find that really odd and inappropriate. It's not a show - there should be discretion when deciding whether or not to attend a funeral. I sort of feel it's disrespectful to attend an event where people are grieving if you have no personal relationship with the star of the show or a 1 degree of separation. But my family has always had private, family-only funerals, so that's how I was raised.

This is pretty simple for me, as DH will not go anywhere that BM is.

SD26 will have BM and that whole side of her family for support, there is no need for DH to attend. Especially, since BM's mother was not that nice to DH in the first place.