Well, first I think your sister is being very cruel by flirting with him and basically egging him on. And he's being an idiot for falling for it. He shouldn't be influenced by her "slutty" behavior which it is since your sister KNOWS he's your boyfriend, and acting the way she is really IS slutty behavior!! Shame on her. And shame on him too.

I don't think I'd want a guy who is so influenced by a woman who gives him a 2nd look and flirts with him a little OR a lot. Who's to say he won't take her out to the barn and have some fun? I'd want a man who wouldn't give her the time of day - who would be true to ME. I'd throw him to the curb, and then go beat up my sister! (just kidding).

Just my opinion, but I wouldn't want him. God bless & take care. Hugs, Lee