What should I do? I want to avoid having my mother-in-law move in during her weight loss surgery.
Why don't you want your mother-in-law to move in?
She's lazy and not serious about the surgery. Also, she's rich enough to get her own place.
You should help family out when you can. You could ask her to find a place to stay after a short stint with you.
That'd mean spending more of my precious money on her.
It's important that you are there for your mother-in-law during this time. It's not so much about her needing a place to stay if she can afford her own place. She may just need help getting things done after her surgery.