I had trouble at first too until I noticed the ear looking a little suspicious. If you create a diagonal obstruction from the top of the hat, to the nose, you are left will only the mother-in-law; the ear has now become an eye.

Once I'd seen it once, the mother-in-law is now prominent. I can still see the wife if I concisely choose to, but the mother-in-law is now the default, strange huh?