Once the vultures find out your mom has this money, she will find herself surrounded by all kinds of potential "financial advisors." If you can talk her into it, have her set up a revocable trust with you and/or your sister as trustees. That way she can gracefully decline all the "help" she will be offered. She can tell these folks her kids handle the money for her and they would need to talk to you. This will also avoid probate when she passes, and the money will come directly to the heirs without the cost or delay of probate.
If she is naïve about money, someone needs to run interference for her. Otherwise, she will be sold every high commission insurance policy and annuity out there, and even shadier folks will try to get her to invest in their schemes. Sadly, churches are common places for these parasites to meet their victims. In your shoes, I would sit her down and get this settled before anyone else gets to her.