I'm going to assume you are referring to a parent sponsorship app. You may want to provide more information when you post if you want an answer.
1. No, your sister is not your dependent.
2. If she is 19 or older, she is not a dependent of your parents either and therefore is not included in the family count. If she is 18 or younger, she is still a dependent of your parents and is therefore included in the family count.