A foot massage is kind of an intimate thing, and if I were your wife I would be upset as well. Perhaps your wife is afraid you and her sister will be massaging other body parts soon. There are other ways to pay someone back for doing a project, like making a cake or buying you a CD.
If your feet hurt why not ask your own wife to massage your feet? Or buy one of those electric massagers. Or sit down. Or buy some better shoes.