I'm going to tell my sister's bf that she is cheating on him
Why are you going to tell him that?
I don't like this guy, I want to make him feel terrible and insecure.
Even though your sister is in the wrong for cheating, you shouldn't tell on her. It sounds like you are doing that just to cause him pain and that is wrong.
If I tell him, maybe he will dump her and she will be single
I don't think you should be happy about intentionally hurting this guy. You should talk to your sister about this first.