My friend is mad at me about something that she should have never been involved in, help?

[Olive]

So, for the sake of keeping her anonymous, I'll use a fake name for her, which is "Panda".
So, Panda is mad at me over something that is strictly supposed to be my family's business. My cousin (whom Panda has only met once and has only texted her since then but claims she "knows all about her") got herself into trouble and has to be taken to counseling. Panda doesn't agree with what we are doing for my cousin because we don't understand her. My cousin is a compulsive liar, so don't you think it would be easy for her to lie to Panda, especially over text? She says we don't understand my cousin, but Panda has only met her once! Besides, I have gone through this same thing with my brother, so I know more about what's going on with my cousin than Panda does.
The fact is, this is none of Panda's business and she started this fight by texting me, fussing me out about it. Even if I did agree with Panda, there's nothing I could do to change what my aunt is doing. Panda is now posting mean things about me on Twitter, and she knows that I know she's talking about me.
She has been fighting with me more and more lately, even before this, and my mom says it's because she's jealous or insecure. She used to be my only friend and used to help me with my anxiety issues, but now that I have more friends and am not having mental breakdowns, she feels threatened. I never tried to replace her with these friends, so I don't know what's going on there.
The only reason I have been emotionally withdrawing from her is because of how she keeps fighting with me over the stupidest things.

Extremely long story short, this coming week is the last week of school. I have to see Panda quite often during the day, and I know she is going to act hateful to me. She'll most likely ignore me and avoid me, but still sit close by to make sure I see how mad she is. How do I deal with her when these next 4 days are supposed to be the happiest of the year? We have the same circle of friends and they are all going to wonder what's going on and I'm going to have to explain it.
When she first started the fight, I told her it was none of her business and stopped responding. I haven't said anything to her since. The only thing is when school comes again, I have to deal with her in person.