I'm worried about my sister's relationship

So my younger sister is 17 and her boyfriend is also 17 they have been going out since October 2012 and were friends since July. The thing is I'm scared he is only after one thing. Whenever he texts her the texts quickly turn sexual, because they are both in school and my sister has a weekend job they don't see each other often they have met up 6 or 7 times since October and each time they're extremely sexually with each other. They haven't had sex, their both virgins, but lets just say that he has reached 3rd base. Another thing is they have never talked on the phone don't loved up teens usually call each other all the time? but they never do. He also said he loved her after only a month going out but I feel like he hardly knows her as they barely do any real talking. Also on christmas he didn't give her a gift until she gave him one the same thing on valentines day. He didn't even call her on valentines day I mean really?! really?! I know I should mind my own business but I don't want her to get hurt do you think he actually cares about her?

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