Cats can be a bit offish with some people. It can even be the persons tone of voice, or if they try & keep eye contact (VERY rude & threatening to cats!!) It could also be her perfume...one of mine hates 1 of my visitors because of the scent she wears.
An idea if she's not already doing it would be to avoid eye contact, also to make sure she smells as much like you as possible (say, to wear one of your jumpers) and a game I've found even the most senior cat to be unable to resist is an entire box of plastic drinking straws tipped on the floor (not sure why!)
Don't forget cat mint if he likes it...put it everywhere, usually helps to chill them out a bit.
Hope it helps, it can be a bit embarrassing when your own cat is scared of the mother -in -law! Good luck.