Been here for a while but name changed specifically for this post as it is sensitive.
Yesterday I went for a family dinner at my sister's house. There were quite a few of us there, all female except for bil. When we finished eating, we all went to the kitchen to clean up and ds1 (5yrs old) and ds2 (3yrs old) stayed in the lounge with bil. Next thing I know, ds2 comes to the kitchen with a sweet in his mouth and says "uncle X said if I smack your bum I will give you a sweet." Ds said this in front of everyone. I was shocked and so was everyone. No one said anything. I then heard ds1 crying so I went to check why he was crying and he said it was because "uncle x hurt my hand". Bil rolled his yes and said "yes yes ds1, I hurt your hand but I didn't mean to."
I just sat down and cuddled ds1 to get him to calm down a bit. As I was trying to comfort ds, I said something along the lines of it being an accident to which ds said, "no it wasn't an accident. Uncle x squeezed my hand." Bil didn't respond to this accusation. At this point I was so shocked I couldn't even think straight so I just picked up my kids and said let's go home.
I'm so upset by this and don't even know how to deal with it. My sister and I are very close so obviously want to maintain that relationship but at the same time I can't let what her husband did go.
Just to clarify, dsis and bil have been married over 15th are and nothing like this has ever happened before so I didn't have any reason to think that me going to the kitchen for 10 minutes would potentially cause this to happen.