How should I tell my sister her boyfriends scum?

He doesn't live with us but he was visiting for the weekend and she went to drop her kids off at school and I was brushing my teeth and he came up from behind and hugged me and squeezed my boobs and kind of creepily smiled at me in the mirror as he squeezed my ass!!!!! I actually feel so bad for her because she is going through a divorce and her husband cheated on her and her new boyfriend makes her so happy and she is constantly talking about how nice he is 😠😠 before this I actually thought he was a pretty cool guy. I just don't know if she will believe me if I tell her, I didn't think he was like this but I don't want to be the one to break her heart cause she's soooo in love with him. But now I just have to awkwardly avoid him which is actually so hard 😩😩