You can only use the key on the account you first used it on. You can try to contact EA to see if they will take it off the account to put to your other account. But you can't have it on your account and your sister's account. You would have to buy another copy for your sister unless you want only your sister to have it and not yourself.
As already stated, you can't have it on multiple accounts with one product key. Each account has to have a product key that is unique to that account.