Why did my sister stand up for me when she has made it very clear that she hates me?

[Rachel]

I'm 16 and my sister is 18. We used to be so so close just a few months ago, and I look up to her. She's such an amazing, funny, caring person and I want to be just like her. We also used to have the best time together and would laugh about everything. She was the only person I was willing to vent to when I dealt with a really bad episode of depression and self harm and my eating disorder relapse (all around last semester). We got into a little fight in February because of something stupid I said about how she's treating her boyfriend. Now that I think about it, I was being a real jerk. I apologized and we stopped fighting, but ever since then nothing's been the same. She took this freshman in our theatre class "under her wing" and it's really hurting me. They tweet at each other things like "I love you big sissy!" and "I have the best little sis ever." And the other day there was this thing on twitter about wearing yellow for self harm awareness, and my sister tweeted, "wearing yellow today to support my little sissy, hannah!" (the freshman girl). And that hurt me more than anything else.

Anyways, today my sister, my best friend, my sister's boyfriend, and I were at Starbucks after school. My sister wasn't really making any conversation with me, which she hasn't really done since our fight. And then this b!tch from my school came up to us and said to me, "How's your little eating disorder going? Probably not very successful, because you've gained, like, a million pounds; I can tell from a mile away!" My sister lunged across the table at her and punched her in the face and screamed, "What the f*ck us your problem?"

We ran out after that, not wanting to get kicked out. But she still won't talk to me. I don't know, it just seems a little weird that she would stand up for me now when she hasn't done it before in the past two months. Any thoughts?