Cheating brother in law 😡

Sooo last night im at the bar having a drink with my friend and I see my sister husband walk in ,I didn't think nothing of it because its a bar duh okay than I see a young lady walk up and start talking to him ok again I didn't care again because hey everyone talks.

I starting talking to my friend when I see him and the girl start hugging and kissing like she was his wife I mean tongue and all 😱yes. The bar was kinda crowded so he didn't see me at first so when me and my friend left I made sure he saw me leaving.

I wasn't go say anything to my sister but this morning my brother in law called me questioning me on what i saw and he's scared ima tell my sister and he should be.

I love my sister to death but I don't wanna hurt her but I feel she is going to find out anyway especially if he's doing things in a public place

He even had the nerve to ask me what was I doing at a bar a so far from my home. I really don't like him now

Edit. Thanks everyone I'm going to tell her you guys are so right I'm just scared that I will hurt her but i'm telling her today

Update. Thanks everyone yes I told her she was happy I did because she said his attitude had changed and we were up all night talking and crying and when her husband came home she did ask him and he said ima bitter lying bitch. Now its a whole bunch of drama and my husband is upset with my in law being so disrespectful 😡