Quote: "she acts mad, crosses her arms, and slams the 360 (with MW2 inside) in the door."
... and you let her get off without any repercussions? No screaming? No threats? No bludgeoning with a hockey stick?
Quote: "Says it's my fault and I shouldn't have had the 360 between the door."
No offense, but it sounds like your mom isn't very bright. Yes, it is partially your fault in a very small, insignificant way. MAYBE like .001%. The rest of it is, quite obviously and undeniably, your sisters fault for slamming the door for no reason.
Quote: "Now I'm out $60, Modern Warfare 2, and still need another $60. I guess I'll just make due with CoD4 for the time being."
Wouldn't it make much, MUCH more sense to make your sister pay for it? Or, if she doesn't have the money, take something of hers of approximately equal value and "accidentally" step on it. Then when she asks your mom to replace it, tell her that its your sister's fault for leaving it on the ground. If you REALLY want to get even, dont just step on it, trip over it. Then your sister will get in even more trouble for injuring you.