I'm pretty sure my mother in law knew, too, but I don't think she encouraged it. If anything she likely told my wife to get her shit together and stop sneaking around, but didn't go out of her way to let me know what was happening.
I have mixed feelings about it. I know (think?) she loves me, but she of course loves her little girl even more. My wife was almost a vegetable. She suffered a tbi when she was left alone on a bed and fell off at 4 months old. She had constant seizures and would go into status epilepticus almost daily. That's where you have back to back to back to back seizures and don't come out of it. She wasn't supposed to be able to walk, talk or do anything a normal person can do.
Well, her mother refused to give up and went to many different doctors and hospitals to find help. She worked with her tirelessly. The seizures eventually went away and not only that, she graduated high school, grew into a beautiful young woman, got a good job and became very independent. She was also a bit spoiled as a miracle child. They're very, very close.
So I get it. She'd do anything for her daughter. Sometimes to a fault.
After 25 years seizure free they started coming back, but only 3 or 4 a year, and mostly absence seizures that pass within 30 seconds or a minute. Just frequent enough that she hasn't driven in over a year and a half now, tho.
So I'm not sure how to take it. There's no malice or ill intent toward me, but it still feels like a betrayal. Otoh her mother has always had issues with saying "no" or meting out punishments, tho from the stories and also knowing my wife she was a very low maintenance child as far as behavior and staying out of trouble goes. She didn't do much that required much disciplining. Always worked hard in school and got good grades. She's a gifted musician. She has some cognitive blind spots, but she's almost like a savant in other areas. She's wired differently than most of us.