Some people are willing to pay an even higher price to have a relationship with their family. Your wife really values this relationship. So air your grievances with your wife. If you confront her sister, it may jepordize your wife's relationship with her sister and that will hurt your wife.
Discussion
Wouldn’t confront the sister, but feel the need to just ask some questions to the wife. Ask her, does she think it is ok for her to take advantage of us (her) like she does? Thank you for your honest comment and opinion.