Background: We're a FatFIRE couple (husband works strictly because he wants to) with a pretty big stash, mostly thanks to lucky stock purchases. Husband's parents are financially set (pension), my dad is set (inheritance), but my mother...
She bought her house late in life, and despite trying to pay it down ahead of schedule, she still owes $350k on what is maybe a $425k house. (Home prices have been stagnant in her area since she bought.) She's always been... sorta okay with money. Not wasteful, but not exactly careful or frugal either. She makes a decent amount at her current job, but she doesn't like it, and it's starting to physically wear on her (she's over 70) especially once she got osteoporosis. Then she caught Covid at work. Her boss thought Covid was a hoax, he didn't take precautions seriously, he let a driver come to work who was visibly sick, and now the majority of her office has tested positive. She's on the mend, thankfully, but she didn't like her job BEFORE her boss wantonly endangered her life.
Which brings me to the titular plan. Husband and I are thinking of buying her house and renting it back to her at the price of property taxes, which would allow her to retire. Her other options would be to either keep working, or sell the house and either buy a mobile home locally or a regular house several hours away where property is cheaper. She's collecting social security, which would be enough to cover all maintenance, utilities, daily expenses, and our break-even rent.
Why we're considering this: It will let her retire from her current job. It would keep the only local grandparent near her only grandkids. It would keep her nearby me, for all that inevitable work kids have to do for their parents when they get older. It would keep a cheap roof over her head later, in case she ever has serious medical expenses that would have forced her to liquidate her assets and lose the house anyway. She would have time to settle her mother's estate (a house and everything in it has to be sold to cover debts, and likely nothing will be left). She could always take a part-time job to earn additional money, something she's done before.
What isn't great, but we're fine with: We could buy her house with cash, which means that while we wouldn't be paying interest on a mortgage, there's an opportunity cost we're paying since money is tied up in a house. Since we're not charging a "market rate" on rent, this would be, for financial and tax purposes, a second home rather than a rental, which means we couldn't deduct any expenses or maintenance, and we'd have to ask her to cover that so we're not actively losing money on this. The house itself is in okay shape but would probably require some retrofitting at some point in the future to fix up the foundation.
What we're afraid of: Worst-case scenario, this is just the first step in supporting her financially and we'll be on the hook for more later. I'm worried that as she ages, her mind and personality will slip, and she'll either start spending more money than she should, or see us a cash register to solve her problems. I'm worried that any contract or document that we draw up, stipulating that she has to cover maintenance and utilities, will be dishonored or discarded later because it's "family". While we can afford this deal going bad, I'm going to feel wretched if MY mom ends up causing my husband problems.
So, am I missing anything to consider? As far as I can tell, this could go great, solving her problems at a manageable cost to us, or it could blow up in our faces. Any thoughts or advice?