Thank you, once again for your insights and suggestions regarding this delicate party situation.
The party is tomorrow (eek!) so I will update again once it's all done and dusted.
I had hoped to be able to speak to SiL quietly about the issue, but when I was able to broach the topic, I wasn't able to get any farther than "I really don't want any drama or awkwardness at the party..."
before she interrupted: "Oh, I know. I've spoken to [BiL]*, and let him know that both [Friend] and I will be there, but that it shouldn't stop him attending if he wants. I'm more worried about [her brother, BiL2**] causing a scene."
I replied (desperately wishing I had a stronger spine than I do!), "If anyone causes a scene or can't be adult enough to put it aside and enjoy the party, they'll be asked to leave."
DH came into the room at that point, and agreed.
(*SiL and BiL are still trying to remain friends. He's a bigger person than I would be under these circumstances!)
(**slight BG: BiL2 is the eldest sibling, and quite opinionated. He never liked BiL, but I think his disapproval over the situation outweighs how he felt about BiL. If we ask him in advance to be nice, he'll likely sulk and try to drag the mood of the whole party down. That will NOT happen tomorrow!)
So. This is where we stand: SiL and Friend attending and BiL "maybe".
Weather permitting we should have enough room to keep everyone seperated if need be
I think the friend is not a friend so should be disinvited, and both SIL and BIL "deserve" to be there since they are the aunt and uncle of the guest of honor. I would assume they would be able to put their differences aside and focus on their nephew for the party. If they cannot, they should be asked to leave. I am a fan of assuming people will act appropriately and making this assumption understood.
Yes, and this puts me in mind of what I was kind of thinking as I read your OP - Yes, you know that your SIL and BIL are having marital issues. You know she cheated and you know who she cheated with. However, I think that in the context of the birthday party you should maintain the polite fiction of ignorance even if they know you are aware of what is happening. Whether they want to attend as a married couple or not is up to them, but I think having the friend either way would be a mistake.
The two bolded quotes are where I'm sitting right now. I don't know (or care) how long the relationship
has been going on, who started what or whether they see a future together, and I don't care if the whole world knows what's been going on, but as long as everyone
can behave at the party I'll be happy. Both sets of DS's grandparents are on alert to keep the peace if DH and I are busy (cooking/wrangling small children), but I truly hope it won't come to that.
It really was highly inconvenient of them to announce all of this right before DS's party!