My mother in law is inheriting somewhere between $400,000 and $1,000,000 in about six months. Her goal is to have a steady guaranteed income for life. She is mentally disabled and has asked me to handle her investment accounts. I'm flattered, but out of my depth; while I am on the lean FIRE path, I'm extremely early into my journey with a net worth of $30k.
What I'm considering investment-wise is splitting her money 40% US mutual funds, 40% international, and 20% treasury bonds. Then we would use the 4% safe withdrawal rate once a year into a savings account, which would be transferred to her checking account in weekly installments.
I'm interested in feedback on this strategy, but I'm losing sleep over legal aspects. One option we've discussed is keeping all the accounts in her name and having a signed agreement that I am allowed to manage her accounts, but I wonder if setting up a discretionary trust is worth the hassle? I'm also not sure how to minimize her taxes, both on the initial inheritance and the yearly gains; I use my Roth IRA religiously but that experience is unhelpful in this huge lump sum situation.
I'm sure there are a hundred things I don't even realize I haven't considered yet and want to be prepared; I still have time to do plenty of research. MIL is not interested in hiring a professional to handle her money (that was my first suggestion). Thanks in advance for any help/suggestions!
(Edit to clarify: MIL is 50, we're late 20s.)