Hey y'all I'm 17 and the girl in question is the first girl I've ever kissed

. Apparently I can't post in the relationships forum because I don't have enough posts.
So anyway I meet his girl, who's about an 8 with a pretty face, on FACEBOOK (haha!) and she tells me I have an amazing personality, which I do, duh. At the time I wanted to get with a different girl instead. A couple months later that one has fallen over (you can't have everything) and I go after facebook girl instead. I end up meeting her in person at a party, I isolate in the back yard, we talk a bit, I go "I really wanna kiss you but I'll feel like a slut ..." and two minutes later kiss her.
That line officially has a 100% success rate

.
Some girl has a seizure in a timely fashion (!) and we leave, I didn't want to make out, just kiss, so this is cool

.
Anyway I have met her a few times on dates of sorts since then and made out, she's really nice and she was crazy into me but she lives about an hour away by train. Also it's exam month so I am busy and I see her 3 times in 3 weeks.
Now, I invite her to a party thrown by a friend of mine. You're meant to only bring +1s but she's like "can I bring a friend?" and me, thinking it's a girl, of course say yes. So it turns out ...
... that this person she's bringing along is a gay friend of hers who she promised to hang out with. I'm like, cool, whatever, but he's a jerk and a big sister type who keeps talking with her and pulling her away when I turn my back. Later on, she says he wants to go home, with her, cause he doesn't trust me, or else he's going to stand out the front and wait for her to leave! Jeez man you kiss one girl and every douchebag thinks you're a player.
I try befriending him a bit in true Mystery style

but I can't get him on my side.
I think, well I'm not going to force her to lose her friend of 10+ years just cause I wanna stay at a party. I'll just see her tomorrow, right? I mean I don't plan on getting laid anyway, lol. She tells me she'll see me the next day and she apologises profusely for it, then she goes home.
So, next day, I wake up at 2pm, send her a text which she doesn't reply to, which is rather out of character. I say hi on msn that afternoon and she doesn't talk, and when I say I found the situation funny, she says she's really angry right now and logs off. I can't get anything out of her further. The next day she won't reply to me either. So I'm a bit worried I guess. I don't want to lose this girl just yet.
so
A) What do you guys reckon has happened here?
and B) How do you deal with a gay big sister as an obstacle?