Hi, my name is Lydia. In my family, we like to compensate each other by returning the favor or doing something else nice. For, example, my father-in-law has been such a help in the past with electric work, etc., so my husband and I take him out for dinner, or get him something we know he needs, and can use. I would find another way to compensate your sister-in-law. Sometimes family may feel awkward accepting money as payment when they are helping out. Buy her some bath stuff, jewelry, or something she is really into at the time.

Hope that helps!