Potential issues with wedding party

Agreed- act like nothing is wrong and keep moving forward. We have 2 divorced couples in our party and everyone is fine with it.

As uncomfortable as it may be, it sounds like you need to sit down and have a conversation with them. Tell them that while you don't want to poke your nose into their business, you need to know that it won't cause a problem on your day. Include your FH as the husband is, i'm assuming, standing up for him. If they agree that they can put aside their differences for your day and everyone is comfortable, there is no reason not to keep them. As long as you are clear in your expectations and they agree to be adults about it, everything should be fine. On the other hand, if that is not the case, you need to know now so you can get people who do. Also mention your concern about causing more stress on their relationship. As long as this couple knows that you are looking at this from a place of respect, they will likely remain good friends, whether they end up standing up for you or not. Communication is key.

I have a divorced couple who is MoH & BM. they have moved past it but they were not always so kosher. Do be careful to insist that any hostilities will need to be set aside for your day.

Oh yeah I get that, my FH almost didn't want to invite his own brother to our wedding because he was cheating on his wife. Does your FBIL know that your FH knows what's going on? I feel like if he says something about it or removing him from the party then that may cause more problems.

"That would take away this theme unless we had another couple we brought in to fill that gap and just found her an escort."

People are not a theme or props for your event.

I'm sorry for the troubles your FSIL is going through in her marriage, but try to think about that separately from your wedding (because the two are not at all related). Be there for her in her time of need and maybe ask her how she feels about keeping her husband in the wedding party. Her feelings should be the guiding factor here, not any "theme".

I'm sorry but a wedding party isn't a "theme" they are the people nearest and dearest to you. Adding a couple just because they fit your BP "theme" is just wrong.

Wow, this is really hard, I'm sorry it' something you have to worry about!
It sounds like your theme of happy couples is a bit out the window, I would drop it entirely, it might make FHs sister a little sad to think about and doesn't seem super necessary.
It sounds like you want this other couple in your wedding no matter what happens with the sisters husband, and they offered without you asking, I would tell them "Hey, we thought more about it and we really value your friendship, and we've grown a lot closer these past few months, we would love for you to be in the wedding no matter what happens with all the other stuff, we should have asked you both in the first place!" 5 months is enough advance, not a ton of time but enough for sure. As long as it is clear you aren't replacing anyone, which would be rude.
I would sit down with the sister and husband maybe a month or two before the wedding and talk out what is happening (before his suit is rented if possible), just like a PP said. Before you do that, I would have FH and the sisters husband sit down separately and have a talk. When you say your FH doesnt like his behavior, if that is alluding to anything serious, I 100% see why he would not want to have him in the bridal party. While it may cause drama if they aren't officially divorced, if it's very clear it's going to happen and be messy, rather than there's a chance it could happen...I would ask the sister if she'd be ok if he wasn't in the bridal party, then scoot him out. If it's just a chance they could divorce, and your FH and the husband are just having some drama, and the husband and the sister are only having drama nothing mentally or physically abusive...then sit them both down and ask if there's going to be an issue at the wedding.
I wouldn't replace the husband with anyone if you do kick him out. Have the sister walk up alone or with the ring bearer so there isn't any drama there or anyone feeling like a second choice.
If it's clear this guy isn't going to be family soon, and is someone who won't be a friend soon due to his behavior, I say scoot him out, you don't want to look back on wedding photos with your FHs sisters mean ex in them. If it seems like FH and this guy will still be friends after all this, and its just drama, then keep him in.