fuck that bitch, shes fucking your shit up, get rid of her and if your friends are your friends they'd kick that girl out of your group of friends, go talk to your sister and just talk to her, i mean shit shes your sister whatever happened will get resolved in the future anyway, and thats that, then just go to your mom and say everythings taken care of, that way you dont apoligize for shit you just talk to her, things will go muhc better