My sister took my best friend to the movie instead of me, I was really sad. 
That would make me sad too.  Was there any reason she took your friend and not you?
She was upset with me so to get back at me she did that.
Lol, well thats rude! Hopefully you guys are better now!  Little grudges like that stink, I like to just put them behind me and move on.
I try to make peace with her but she is always angry so I let it be.