OK, so here's what we know.
Ownership: mom's
Objectives: insurance first, inheritance second
This could get messy in a couple of ways:
1) Siblings think you might have done something shady to be named beneficiary (e.g. emotional pressure) and they challenge your inheritance of this money.
2) Some optional medical/life expenses come along, such as a risky and expensive surgical procedure that might extend her life a couple years (real-life example, my father-in-law had open heart surgery at 88 years old. In hindsight, it was a bad call.). Suddenly, those who stand to inherit the money are torn about what to do and feel entitled to opinions. The people who don't want to put her through medical experimentation get in a fight with those fighting to save momma.
3) Mom's will says divide everything evenly or percentage-wise among X people. Oh, and you're beneficiary on this account. There is a dispute about whether the entire estate is divided evenly or the entire estate minus this account. You end up with less of the rest of the estate than your co-beneficiaries.
I suggest putting it in a joint brokerage account, AND having mom update her will to specify exactly who gets the contents of this account, and how the account factors into any other divisions. A lawyer is necessary to get the wording airtight.
For asset allocation, I think 60% stocks 40% bonds would be fine because near-term volatility reduction is not an objective, but leaving a nice inheritance is. Also, bonds aren't as low-risk as they used to be.