It's DS's 1st birthday in a few weeks time. We are inviting close family on both sides and a few friends, we get on with all of them which is fine BUT on DH's side it's a bit complicated... His sister got married this year to a guy who went through 'a phase' of controlling her, he smashed her flat up and beat her up about 4 or 5 years ago. DH's whole family hated him but he managed to crawl back into DH's sisters life and sweet talk his now inlaws (he is one of those very charming types on the surface). He has gone for treatment and to be fair, he seems to be trying his hardest to put all his issues behind him now.
DH didn't talk to him for years but agreed to be civil to him due to him marrying his sister. However, DH's brother & his wife & kids have never even met this guy out of principle. DH's sister is great with our DS and loves seeing him, as does DH's brother and wife. So they are all invited to DS's party as it's his day and I think all the tiffs should be put aside for that day.
However, I've not said to either DH's sister or brother who else is invited (but assuming they may guess?). I don't want things to be all awkward at DS's party so should I tell them who's coming and to behave themselves for sake of our DS?
Yes, I think you should tell them, and ask them to behave themselves if he shows up. Otherwise they might be shocked and be thrown off guard, and that would be awkward. Hopefully they'll just ignore him and get on with lavishing attention on your DS.
We had a naming party for my dd, and invited both sets of grandparents, none of whom are married to eachother any more and rarely speak. But they were prepared in advance, and even made the effort to be civil, and we all had a great day.