Claire - you're falling into a stereotype. Yes, I know sisters are supposed to behave like that to brothers, and brothers are supposed to expect that. I know that given your unusual history together it might even be seen by both of you as affirming reality, but it doesn't have to be like that. Those stereotypes are best left in SitComs and rancid old movies.
You can be nonconforming to the stereotype, and still not be outcast. So can your weird brother, who's had to live much of his life being different, what with the loss of his hand and all. His act of rebellion is not to kowtow to society, and wear a synthetic flesh covering, but to be out'n'proud. To make a virtue of necessity, throwing it in the face of all those who teased him about it.
Not every girl in your situation has a brother who sees her as just a pesky teasing sister. You should thank your lucky stars for him, pain in the petunias though he may be sometimes.
Sticking out your tongue like that is permissible. Being catty is not.