Is it normal that i'm a little bit jealous of my boyfriend's sister?

I've never been through anything like this and I don't usually give these things much thought, but I caught myself experiencing a feeling I've never thought I would. My boyfriend and I have been together for almost 6 months now and it's the first time I've ever been in a serious relationship. He has a sister who's 2 years younger than him and 1 year younger than me. He says he loves me very much and treats me in a way I've never been treated before, but since I always seem to be looking for something to worry about, there's something bothering me about his sister. She's extremely quiet but quite sweet and treats me well. The thing is, it kinda seems to bother me a little bit when he says he thinks she's very beautiful (but it's never out of the blue) or when he seems to pretend there's nothing wrong with the way she lives her life. She's 18 and the only thing she seems to really care about are parties, her friends and going clubbing, she doesn't seem to be very interested in studying or trying to get into college. We look nothing alike (I'm a blonde and she's a brunette) but my boyfriend's always told me that I'm "the most beautiful girl he's ever seen" and I don't really care about that (though I'm always grateful for the compliment), but even if to him I'm the most beautiful girl ever, it always bothers me when he comments on his sister's appearance. I feel terribly awful when I think about all this because it's stupid, petty and very unnecessary. I know he thinks I'm prettier than his sister, but it shouldn't matter to me for she's his sister and will always be. Anyway, I'm sorry for such a long description. Many thanks to anyone who tries to help me out with this one.