She wants to be free to be her own person and make her own decisions instead of taking care of her sister's kids when she is home. My other question is how much responsibility is too much for an 18-19 year old. Although her family could have legitimate reasons for trying to prevent her from moving on with her life, unless she is mentally or emotionally unbalanced, she can do as she likes. Most families try to delay the time when children must assume all responsibility for their lives, since it is so difficult to send the ones you love out into the harsh world. Whether or not your friend is capable of taking on the responsibility for herself is a question I can't answer.
Many young people do assume responsibility for themselves at eighteen.
It sounds as though your friend resents having to help her sister with the kids. It could be her sister is taking advantage of her, or it may be that she feels this is the least your friend can do to repay her for taking care of her. However, at nineteen, she should be allowed some say in her life. Your friend can choose a couple different approaches. One is to try to win the support of her family.