My husband flirts right in fron t of me?

Sometimes i feel as if my husband is flirting right in front of me. We have only been married 6 months and we are both in our early 20’s. I have a 15 year old sister that is very pretty. She comes over and stays with us on the weekends some. It literally makes me sick to my stomach when she is here, because they are constantly flirting. They pretend to be fighting smacking each other around and he even smacked her once. Then when we go out with my girl friends, he talks to them and ignores me. I don’t know if maybe I am making a mountain out of a molehill or if I should really be worried? I just want to know why he would do this?