In my opinion, if your sister applies for a student visa and you become a sponsor the only quotation may arise if your sister is still dependent on your parents!
At the time of 143 visa application processing case officer will ask you to provide dependency evidence. If your sister financially relies on you and she does part-time work it may be a little harder to convenience the case officer that she is still eligible as a dependant. I guess the best solution is to ask your parents to become sponsors and pay all the tuition fees and living costs for the duration of the study. Remember to keep those evidence as you can provide them as evidence to prove dependency. Also making sure she did not earn a full-time income at any period of time. otherwise, things may be a bit complicated in future! I always prefer to keep things simple and explainable.