The problem is, I am having sexual fantasies about her mother. If your mother-in-law is attractive, you are seeing her as an available female instead of as Mom. You need to recalibrate your impression of her, and you can do that by making a concerted effort to substitute more motherly images in your head.
Is it common to fantasize about getting kinky with your mother-in-law? (or if you are female, your father-in-law)? I find my GF's mother quite