What do you think?

My boyfriend and I have been going out with eachother for 6 months. Lately he has been inviting his little sister (whom is 20 years old) to go and sleep at his house or she invites herself to sleep at his house while I'm there. It kinda gets me mad, for I want to spend time with him not her. They talk and play around with eachother. One weekend when we went to a rodeo she sat in front with him and I in the back (which I think its odd for her to do that) they both were talking and I couldn't hear anything for they had the radio on full blast so I eventually fell asleep. I'm getting sick and tired of this. Maybe she is jealous that I'm going out with her brother or she is jealous that she doesn't have a boyfriend...she tries desperatly to get this guys attention (which kinda makes me laugh) she is a very beautiful girl. But how can I let my boyfriend know how I feel about this that I don't like her spendin the nite over when I'm there or should I just shrug it off and let them be 2gether