I know this wouldn't happen to any of you but let me give you an in-law nightmare that is happening now to my sister.
Her son married a girl who, at first meeting, seemed sweet. That all changed when they got engaged.
Her mother took over everything, right down to how this boy treated his own mother. They convinced him she was trash because she was single and had two boyfriends.
When my sister came into a good settlement with a former employer, she paid off bills and treated herself to a boobjob. The girl's reaction: That money should have gone to (the son) to help pay for the engagement ring.
My sister was supposed to pay for the rehearsal dinner. When the girl's mother did all the arrangements, my sister insisted that if she was to pay, she would do that and did. The mother then cancelled it all, saying there'd be no dinner, but then, behind my sister's back, took the wedding party to a restaurant. The girl and the son were then angry at my sister for not using the dinner money as a wedding gift.
At the wedding, the son read a long thankyou letter to the girl's parents; nothing was said to the mother that raised him alone for most of his life.
Now they are pregnant. When my sister asked her son "will I be part of my grandchild's life", he responded "I doubt it".
The mother in law has a key to the house; my sister isn't allowed to visit.
When my sister called one day, the mother in law answered, went off on her for being trash, then denied she was ever in the house at all and that my sister "must be crazy". The daughter later went off on my sister for "accusing" her mother of such a thing, but used almost word for word the same things her mother did.
Abaya has a good point: Kill her with kindness. Offer to help with things, ask her a sweetly toned "How was your day today?" Stay out of her way if she looks like she's in kill mode. This may not change her antics, but it will give her no ammo.
I too am the "blacksheep" in my spouse's family so I like to rub it in that I am the only in-law she has left(her other two adult kids divorced and moved into her house), plus I gave her her only granddaughter
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Remember this is not forever and soon your life will be your own again.