Words to live by : "Once it's a favor, Twice it's a gig"
Granted your sister is having a tough time, and you're stepping up to help her out - BUT don't let her get dependant on your generousity. People (especially family) get too comfortable with situations and take advantage (most times unintentionally) but you're filling a void in their life that easily gets overlooked especially when filled by a family member. A bigger example is when family lends other family members money - the importance of pay back is not taken as seriously as it would if cash was borrowed from a bank or creditor.
Like you said you're only 16, but you're getting a crash course on life - if you're not happy in the situation or feel you're being taken advantage of tell her (as you did us) and if she can't accept your reasons then it's time for her to make other arrangements - like paying for daycare. Don't let others' guilt trips anchor your life, if you do you'll be a slave multiple times over in your life until you shake the guilt trips.