I love my dsis and we are really close. Unfortunately dsis has a really bad track history of men and the one she is with now is no different. They have been together for half a decade for context. It has recently come out that he has done the very cliche messaging a woman from work thing. Not sexually but definitely inappropriately. To me it looks like he is trying to buy his luck and see if she takes the bait.
So the issue is and that my family don't like dsis partner regardless of this for other reasons such as he talks to her like crap, does nothing but drinks and smokes, is immature and treats her badly in her vunerable time of need during a health crisis. Dsis constantly moans about him and they are never on good terms but for some reason or another she stays with him so we all just get along with it to respect her decision. Until now.
With this new revelation I have decided I am uncomfortable proceeding with our Christmas plans where it was to be dsis and him and dparents and me and dp all get together. I was meant to buy him a gift and have dinner with him. I have told dparents I am not happy with this and although I won't go against what dsis decides and won't make a scene, I don't want to buy him a gift or feel forced to act even civil with him on Christmas day. I don't want it to come across like I am enabling his behaviour towards dsis. I want him to know I very much do not stand for it even if she does.
Aibu to tell dsis I don't want to go ahead with the plans. I know she won't be happy and will most likely find a way to turn it on me. Dparents feel the same.