Okay, in this case I'd say don't punish yourself. Taking away your family members to avoid bothersome moments, just be frank. Hang out with your sister and keep going about your life. If he chooses to hit on you again, confront that. You could choose to say the truth "Don't hit on me, it makes you look like a low life cheating husband." Or be sly about it, "Do you hit on every female you see or do you just like being a grown dick with ears?" (of course I mean this with respect towards you. Also if you are to say one of these or the second one I highly suggest you say it while your sister is witness to him hitting on you.)
By the way, if your sister has a spine, she did not let him live that sentence down. I bet you that your sister yelled at him for a bit of time. You should speak with her and make sure she gets her man in check or she could very well lose him, it sounds like he could have been testing her on who he could hit on and get away with out.
a good chat about control is needed her, your going to have to be the bigger sister and tell her to handle it.