Boyfriend maybe flirting with sister?

I have been dating this guy for almost 4 years and things are ok. I found it odd at first when he would never let me meet his family ( to this day I have only talked to his parents four times tops) but I accepted it. But lately he has been hanging out a lot with his litter sister she is 15 we're both in out 20's. I didn't think much of it before cause I have friend who are very close to their sisters but for the past 6 months or so he will break dates with me to be with his sister and on the rare occasion that he asks me to go anywhere with them its like i've gone out as the third wheel with a guy friend and his date. He is constantly flirting with her and paying special attention to her (the way he is with her is how he was when we started dating) and he pays almost no attention to me. He has even started arguing with her about how she needs to break up with her boyfriend and he just seems jealous. They were never really close before all of this and I was just wondering if it seems odd to anyone else or if im just misreading it since im not used to him even wanting to be around his sister.