That's the knee-jerk reaction to the question. But what if that demon is a gorgeous bad-boy gone straight laced who sings laments to her beauty, cooks fantastic meals, tries to bring out the best in your child and seems to make your daughter genuinely happy, even though you can see Cthulhu under the pretty-skin-mask?
Or even worse, what if your daughter is in love but can't see the blatant monster under the guise? Do you risk confrontation? Are you passive aggressive and maneuver a strike when the time is right? Or, would you try to get to know the thing before making any judgment calls?
Poor Marcus, my main character in my latest WIP, finds himself to be that son-in-law from the Atra Solumn; which is basically hell.
Rehan, Marcus' mother-in-law, reacts rather like a typical protective parent and rightly so. Rehan does so much as to not give her blessing for the marriage of Marcus to her daughter, Platt.
In Rehan's point of view, a demon has seen fit to take her daughter away and she does everything possible to stop our heroine, Platt, from becoming a demon's bride.
If you want tips on how to make a mother-in-law more amicable to you, then I wouldn't advise following the steps Marcus took for his wife-to-be. But the ride sure was entertaining!