Need some advice... Girlfriend's younger sister problem

Been with this girl for a little over a year now... we argue a lot, we get into fights when we disagree and whatnot. But that's normal, right? However, one of my main things I wanted when finding a girl was for their family to like me. Her parents are pretty cool with me and I get along with them fine. It's her bitch ass younger Sister who is the problem. We got into an argument since she's out of state away at College when my girlfriend doesn't call her younger sister that much and then her sister talks shit about me and says things like "Because you're always with HIM" "FAMILY COMES FIRST!!!! NOT NO LAME BOY" and then I got offended by that and told her "Um, It's not my fault she doesn't call you..." and then she bitched at me saying how I "control her" and I told her that my GF makes her own decisions in the end.
When she visited here over the new year, I bought her free drinks, burned her DVD's she wanted, Burned her Word for her Mac. Did all this nice shit for her that I didn't even have to do, but only did it because I want to get along with her and it just so happened that when me and my GF and her and her BF went out one day... We were supposed to go to the beach, but she had forgotten her Swimsuit, so we went to Town to go shopping for a cheap swimsuit. I got real frustrated because my GF was out spending money on all these dresses, when we were supposed to just look for a swimsuit. And then she finally found one, and it cost $200. and I was like "What the fuck? Are you crazy? You don't need to buy a TWO HUNDRED DOLLAR swimsuit" and because I opened my mouth in front of everyone, she felt embarassed. And then me and my GF got into a fight in front of her Sister and then after everything was done, I swallowed my pride and apologized to her sister saying "Sorry for getting upset back there, I was just frustrated" and she acted like everything was all cool. But I could see how she kept ignoring me and not wanting to talk to me. Now the problem is, she went back to College... and she posted a Bulletin on MySpace which I saw about her saying "IT PISSES ME OFF WHEN BOYS who treat my sister/their girlfriends like SHIT!! grow the fuck up!!" and that just got me so upset..now I just don't know what to do, how to handle this situation. I almost feel like breaking up with my GF because of this situation, but I don't feel like it's fair to her. I feel betrayed and wish she could've brought it up to my face, not talk shit about me to everyone while she hides behind the computer. I told my GF that I'm done with being nice to her already. There's no way we could ever get along. I just feel so betrayed when I do all these nice things to get along with her and she just doesn't give me a chance, because she probably feels like I'm taking her sister away from her.

 

BINGO... we have a winner. She just has to learn to accept you. You're dating your gf, not the sister. It's not like you're all living together, or are always together. I wouldn't even stress over it if I were you.

Why not give your gf some alone time with her sister once in a while? Even if she ask you to come along, just say no. A few hours with her sister and you out of the picture isn't going to kill you.