I give my visiting mother-in-law a thin sheet. It's freezing in my house.
That sounds a bit cruel. Why would you do that?
Irrelevant, the question is what am I doing to do now?
Giving your mother-in-law just a thin sheet to protect her from the cold is extremely rude. You should provide your mother-in-law with adequate warmth to protect her from the cold. If you leave her with just a thin sheet she may get hypothermia.
I just want to see her suffer for weeks.
You shouldn't want to hurt your own family, and you need to give her a bigger blanket. What if she contracts pneumonia and dies because of the cold? Would the rest of your family ever forgive you?