I'll try keep this brief but avoid drip feeding
BIL (DH's brother) split with his wife (let's say A) a few years ago, initiated by him. DH and I weren't super close to BIL & A but we both have DCs who get on, and we'd spend maybe a couple of weekends a year with them and have a nice time. They lived in a different country a few hours away.
After they split we of course continued to see BIL and separately I visited A. As far I was concerned they were both family when they were together, and I wasn't going to ditch her just because they split up. When I was there she told me she'd found out that BIL had started seeing his new gf (B) before BIL he'd initiated their separation and divorce. B was also married - they both left their OHs to be with each other.
So to summarise: he cheated on A, decided he loved B more, lied about it and told A he wanted to divorce because she is hard to live with, left A dealing with most of the burden of 5 DCs (2 with SN), and now wants us all to welcome B into the family with open arms.
I've avoided meeting B so far but now there is a family wedding and both A and B will be there. I'm dreading it. Of course MIL doesn't know about the affair and thinks B is "wonderful" and we can all be great friends.
DH thinks I'm being ridiculous: it's none of our business, and also tells me I can't believe A's story (in my opinion she has far less reason than BIL to lie though). Maybe he's right and I'm just pissed off about having to spend time with these people and play nice when my own family Christmas has just been delayed for the third time 
I'm just going to have to suck it up I know. Anyone been in a similar situation?