I need help so i can help my sister who..?

has a disability. she was born 5 months early. so it resulted in sarable palsy. =( these Children typically have tight, stiff muscles which cause rigid, erratic and jerky movements and result in difficulty changing positions and she also turns her left leg in and when she walks does not put her heel down. shes 15 and im 16...i feel bad for her but she takes it too far.people in my family coddle her and do every thing for her...but i know she can do it herself....shes mentally smart and she only has difficulty walking..i understand its not easy for her...but what is she going to do when she gets older? she's just hurting herself! i always get yelled @ for not helping her do her exercise and stuff.. i don't mind but she 15 don't you think she should at least be able to remind me or ask me? my grandmother always gets mad at me b/c she doesn't understand y my mom and dad haven't gotten her a different brace....what do i do i need help! how do i show her she can do stuff by not being mean about it?