In-laws are parental figures in your life who aren't biologically obligated to love you. If anything, they might resent you for taking their little birdie away from the nest, and everyone who has ever had parents or Googled "Sigmund Freud" knows just how complicated parent-child relationships can be.
Mothers-in-law are notorious for being particularly judgemental, and there's a whole subreddit called "Just No MIL" that is dedicated to commiserating and sharing horror stories. A recent thread asked suffering children-in-law what the worst gifts they've received from their MIL is, and the answers are masterclasses in passive-aggression.
Regifted me the necklace I gave her the year before. -Molly_Moon
A hamper full of foods she was well aware that I didn’t like.
A subscription to a magazine I don’t read, and when I was asked I specifically told her that I don’t read that type of magazine.
The most hideous handbag you’ve ever seen (which I’m sure came from a charity shop)
One of those “gift with purchase” sets you get free if you buy certain cosmetics.
And many many more. -Carrie56