i agree with Leed. About both your sister being cruel and your bf.

To be honest, I don't think that a man should be looking at his gf's sister and smiling when she's obviously flirting with him. It's like holding up an invitation to her and welcoming her to do more. I think most men know this too. I doubt he's so dense that he doesn't get it so that excuse is out the window.

The fact that he would act that way and THEN mention to you how attractive your sister is, that's just uncalled for and completely disrespectful to you, especially when it's so early in your relationship and it's obviously not well established yet by what you've described.

He's an asshat and I wouldn't trust him as far as I can spit.