@Kathleen D.
I am sorry to hear about your mother's loss. I don't want to sound like a broken record, but I think it's important to acknowledge the loss and the emotions that come with it. Money management is a very logical thing but emotions can interfere with that. I wish you patience as you help her through this.
I'm not a financial advisor.
TLDR: Calculate annual expenses. Strategy One: Most passive (to my knowledge) is an annuity. Strategy Two: Move 5-7 years expenses into a high interest savings account. Invest the rest into a low-cost index fund.
About your question, I don't think you can answer it until you figure out your mother's trust in you, health, financial education, lifestyle and goals. Then you can help her plan her finances around that. What if she wants to donate a portion of the money? It's easy to come up with a financial plan that looks great on paper. But it takes motivation to follow through on that plan. That motivation will come from her goals (travel, creative outlet, etc.).
Get a clearer picture on her expenses. If you're able, review her past statements and categorize her spending. Identify annual expenses like doctor appointments, insurance, taxes, Christmas gift shopping, etc. Identify unplanned expenses like car maintenance, house repairs, etc. Sum it all together into a total annual expenses number. Then multiply it by 1.2 (more or less) as a buffer.
Strategy One: Buy a fixed annuity with 2% inflation. This is probably the most passive option (to the best of my knowledge). But you need to make sure the monthly income does meet the monthly expenses you calculated. In my opinion, the biggest downside to this strategy is it locks the money up. You can't take your money out of an annuity (again, to my knowledge).
Strategy Two: With the annual expense number, you're able to figure out how much she will roughly need for the next 5-7 years. Let's say that is $50k. I'd move $350k into a high interest savings account. And then I'd invest the rest into a low-cost index fund that tracks the total stock market (VTSAX, FSKAX, etc...). Then reallocate once a year, adjusting the cash savings for inflation.
I like strategy two more because it is more flexible. Life changes all the time, and if your mother decides to use this money differently, the money isn't locked into an annuity. The plan you set up today doesn't have to be the same next year.
Don't forget about taxes! If you want to reduce your taxable income, you could research some municipal/tax-exempt bond funds.