My sisters boyfriend asked to kiss me !!

I don’t really have friends to talk to about things so I thought why not come on here to talk about what happened. So pretty much I spent the night at my sister and her boyfriends apartment, and last night me and him were on the couch watching movies. We were talking, laughing, and just bonding. And out of nowhere he asked me if he could kiss me😐. I said no of course but now I don’t know what to do. I want to tell my sister but also I’m not sure how to tell her, and I’m scared maybe she won’t believe me. Idk but if anyone has any opinions on what I should do I wouldn’t mind hearing them.