The answer to most of your MiL’s investment question is answered by I-Bond. I Bonds are the Treasury’s inflation-protected product. They are currently paying 7.1%. Everyone in the country should be in them. You can only invest $10,000 per SSN per year, but she need to put $10,000 into an I Bond this year and another $10,000 into an I Bomd after the Nee Year and she’s part of the way there. It’s just an insanely good investment. My wife, our mothers and our kids are all buying the max this year and next. If your family is willing, maybe relatives are willing to hold grandma’s money and buy grandma an I Bond.
https://www.forbes.com/advisor/investing/what-are-i-bonds/
For the rest of the money, you might actually look into an annuity. At 89 her annual return will be substantial.
Otherwise, go to Vegas, share a buffet with Ted and put it on red 29.