My two best friends are dating, and all 3 of us hang out every saturday. this saturday, one of my friend's couldn't make it, and me and her boyfriend hung out with my sister. my sister left the room to get her phone, and he tried to kiss me. if i tell my best friend, she'll think i'm just being jealous, but if i don't tell her, he will keep doing it. what should i do?

This is not a law question
Tell your friend and set a trap with your friend

I strongly believe you should tell your best friend over coffee and understand that she might act out in rage I mean she could even say some hurtful things but you have to understand that she’s being backstabbed by this boy who she trusted and if she doesn’t believe you prove it to her by leading her boyfriend on even if it’s over text say things about the kiss so she can see land witness it.