So, the sort version is, my DH's DB has recently finalised his divorce with my SIL, (she and I are the ones related only by marriage IYSWIM). My BIL has behaved appallingly and continues to do so since their separation (no other parties involves, SIL had just had enough), although the reports of his behaviour are all second-hand. He has been unpleasant at family gatherings in the past, e.g. aggressive (in a shouty way, especially after a couple of drinks). Based on my experience of the couple, I have no reason to dis-believe any accounts of the sort of stuff that has gone on, in relation to the kids, threatening to bleed my SIL dry (of money) etc.
So, my DH's family are all on the 'side' of my SIL, have helped her through this period, except for my FIL who is sticking with his son. My BIL is the one who wants to be acrimonious, SIL is trying to keep things civil with all parties.
My FIL wants to heal the rift, (although BIL has cut off his DM and one sister), to this end, we have been invited out to dinner tomorrow night, with FIL, BIL, BIL's girlfriend and the other sister. I have been stressing over this for weeks now, as I feel that I would in some way be condoning his actions by turning up and playing happy families. I do have migraines sometimes and was thinking of feigning illness, rather than ringing up FIL and telling him, the real reason that I don't want to go. It wouldn't be so bad if it were a family day, but a dinner will be quite intense.