Your sister needs to understand the impact this is having on your family. Quite simply, she is being selfish in having you babysit her daughter all the time when you have told her that you need some time just your family. You should not feel guilty about this--she should! It's fine to help out once in a while (I hope you're getting paid for your work--even a little something), but you shouldn't be called on to sacrifice your husband and daughter so that your sister can work. Is there no one else she can call on to pitch in?--no friends, neighbors, people at church, mother, mother-in-law, other siblings? I understand she is your sister, but your higher priority is to your husband and child.
Quite simply, you are going to have to say "no" and stick with it. Phrase it kindly, but talk to her quite sincerely about the detrimental effect watching her child every weekend is having on your family. (Reverse the "guilt trip"--she probably makes you feel guilty for not watching her child, so don't feel guilty about making her feel guilty for being so selfish.) Give her plenty of time, but tell her that you will no longer be able to babysit every weekend (if once a month is fine with you, offer that). Then make plans for the first affected weekend so that you will be away when she wants to drop off her daughter (even if it's just going to the library with your husband and daughter).
Ann Landers once said that you have to give people permission to walk all over you. Others say you teach people how to treat you.