Log on to her account and try to log in, it should say the reason, and possibly the moderator that banned you. Write a ban appeal with her username, her ban reason, and why you wish she should be unbanned.
Get yourself unbanned first, because they dont accept ban evasions usually, considering saying "my sisters account was banned" could easily actually your account that you evaded on. (i didnt say you are evading, but it's a possibility)
Comments on Profile Post by mr_momo_2