Drop by unannounced when the husband is out. Look her deeply in her eyes, tell her you have been thinking of her all day, you just can't get her out of your mind. Then drop trou and take a humongous dump in her foyer or entryway while singing Sweet Caroline. Leave without word, comment, or explanation. From that point forward deny everything and just opine that she is one sick woman in need of professional help. Tell your wife that it is obvious SIL cannot come over and you will henceforth avoid all events where she is in attendance as you do not wish to feed her delusions.