A messy situation has developed in our wedding party and it looks like it may be getting worse. FI's best man is his cousin (I'll call him Bob), and one of my BMs is the woman who was until very recently Bob's FI (I'll call her Betty). The two of them split up a couple of weeks before Christmas. Bob and Betty have a child together so their lives are still attached to each other, and they have been fighting bitterly for several weeks. Both of them have struggled to interact with each other in a civil manner for any length of time.
From their break up until now I had not discussed our wedding with Betty at all. I've just tried to be a good friend to her as she went through the painful first stages of the break up. It has been in the back of my mind though that she might choose to bow out, that it might be too painful for her. The wedding isn't until May 31, so I thought it best to give it some time, and see if things settle down between them. Her dress has been ordered and paid for, so I saw no reason to force the issue. Plus, obviously, Betty has had far bigger things going on in her life than being a bridesmaid.
FMIL (Bob's aunt) and Bob's mother have asked me on several occasions what I plan to do. My bean dipping skills have been pushed to their limit. Finally this weekend I was talking to FMIL and she pursued the question to a point where I broke down and just told her that had no intention of kicking Betty out, if she ultimately decided not to stand up I would understand, but I didn't think she needed to decide anything right away. I wished I had managed to keep my mouth shut, for I suspected that this conversation would be made known to Betty.
So tonight Betty called me to talk about the wedding. She said we are still friends, she is still happy for us and wants to stay in the wedding. She also said her feelings towards Bob are improving (not quite as hurt and resentful), and she thought she could be happy and enjoy the day without the two of them fighting. However, she also said that either FMIL or Bob's mom (I can't remember whom) had told her that Bob didn't want her there. I honestly don't know what to think about that last part. It could be true, it could be an exaggeration or misunderstanding, or it could be completely untrue.
FI has no objections to Betty standing up with me, but he doesn't want Bob to be uncomfortable or upset either. I feel like he should talk to Bob himself to find out how he really feels about it, but I also feel like this isn't really Bob's decision and I don't want him to get the impression that it is.
So, in short, this is a mess that is being forced to come to a head before it should have. Any advice?

Re: Best Man vs. Bridesmaid
Definitely don't do it because of hearsay that Bob or his mother or your FMIL want you to.
To be honest, I don't think your FI needs to talk to Bob about this. Bob and his family need to accept that it's not up to them whether or not Betty is in the wedding, but if she is, they all owe everyone the courtesy of behaving like mature adults and not creating drama during your wedding. If that's going to be a problematic or unrealistic prospect, then you might need security to escort away anyone who crosses that line.
I am more concerned about whether FI should talk to Bob, and if anyone has any other perspective on this that I am not seeing. Should I try to just keep letting it rest where it stands?
Thank you @Jen4948.
To the bolded: Yes. Stay out of it. FI should not talk to Bob. You said that he's okay with Betty being in your wedding party, and it's really only up to you and Betty (depending on whether or not she wants to stay in, and you said that she does).
So it's really nobody else's business. Everyone needs to act like an adult. So, when you get asked about Betty by your FFILs, just tell them, "Yes, Betty is still one of my bridesmaids." If they demand that you force her to step down, tell them, "No." You said yourself it's a shitty thing to do. Let it go at that.
Another aspect of this is that Betty's family makes a lousy emotional support system, and she has grown so close to Bob's mom & FMIL that they have been trying to council her through this. They have a lot of influence over her. If it came to them opposing her position in the wedding party, I know I can stand my ground well enough, but I fear they will try to talk her into stepping down.
I guess there's nothing I can do about that though, and that's just one more point in favor of trying to let it rest.
@OliveOilsMom Good idea about the photographer! I don't think we'll do a ton of posed shots in general so that should be fairly easy to achieve. The seating should be pretty easy too. We're not doing a traditional long head table. With everyone's SOs the wedding party will need to be split into 2 round tables anyhow.
Thank you everyone. I've been feeling like I'm being pulled towards a drama black hole, and I didn't want it to cloud my judgement. Please think good thoughts for me that I can be peacefully allowed to enjoy our wedding with my friend included.