Weak Kitten

I thought my kitten's leg was asleep when she stood up the other day because she stumbled upon standing and seemed weak on it. But then the weakness didn't go away shortly and she wasn't walking right (stumbling, swaying, generally walking like a drunk) and was moving her head kinda weird. She doesn't walk around as much now and when she stands up or tries to walk, her back end falls back down. She has a brother who is perfectly normal. Could it be that our older cat has been rough housing with her too much? They're all outdoor cats that spend most of their time hanging out around my house, but I do feed them. I really don't know what to do, so if anybody could please help or just give any advice, I'd be very grateful. I just want my baby kitty to get better

you need to call a vet as soon as possible. she needs to be seen. this doesn't sound like something that is just going to go away, especially if it has been a day or more and she is worse. good luck and please keep us informed. 

Could be anything, especially as she's outside so young. Please get her to a vet asap. 

No, it's not roughhousing, I'm afraid. Your kitten is seriously ill, in pain and distress, and only a vet can help. It could be a raging infection and that can be treated if you get help for her right away.