If he genuinely did as accused then I CAN see why you would feel so bad at the idea of looking like you condone his actions, by socialising with him,
but it puzzles me as to how the rest of your immediate family CAN, knowing the same as you & want her to get back with him, not with kids involved & them potentially in danger, it just doesn't sound right
- this is what makes me wonder even more so, if the rest of the family actually know more than you do, & that she exaggerated her claims when they were split up, to stop him seeing the kids, hurt him etc etc - I can see why you would hate him, based on what you are told, but I have lost count of the number of times I have seen really nasty accusations fly about the other parent when relationship breakdown - it can all get VERY Jeremy Kyle
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I've seen my friend dragged through court & accused of been an alcoholic prostitute by her ex wanting custody of their Son - at the time he really seemed to believe it, because he hated her so much & wanted to believe it - she accused him of being abusive & the police were called many a time - after she'd wound the hell out of him & he reacted - because she wanted it on record, for HER case - these days they are good friends & he thinks she's a great Mum
I could offer lots more examples, my own Mum included, who regularly accused my Dad as being abusive before & during their own split & divorce- she was actually the abusive one, we as kids only ever saw our Dad raise a hand to her, in self protection - he to was accused of been abusive to my Brother - he was playing with him & trying to teach him to swim - they are back together now & have been for over 50 years - my own Grandparents etc were close by & knew the truth of it & accepted my Dad back without prejudice - hence why I'm a bit sceptical that your SIL is as honest as you think
Have you tried talking to other closer members of the family about this??, perhaps those who live close to them ?? maybe their opinions might help you decide???
But putting that aside, even if he IS as bad as painted I personally wouldn't allow that to affect my family taking part - or let the parents of the Christened DC down by not going -
we had a situation that was a little similar, in that a friends 40th party we were invited too, as had invited someone whom we would normally not pee on if they were on fire, for very good reason too - but we went anyway, because our friend wanted us there, especially important as she had been ill - in the end we weren't the ones left feeling uncomfortable & hiding - the other person was - just go, show a united front & if necessary keep out of his way - but I suspect if the accusations ARE true - you probably won't have too - he'll avoid YOU 