Historical stock prices exist so you can see what the price was on the day of her death. If you have been reinvesting the dividends that would make it more complicated.Getting those values considering all the splits, mergers, etc, would likely be impossible, or totally impractical. We can withdraw any fresh divies and or cap gains if needed, but selling the holdings would be a nightmare.
No need for the money, and any managing/rebalancing needing to be done can be handled using the generated income.
Assuming I die first, DW will get the whole stash with a stepped up basis.
Happy everything.
I had a similar situation with a single stock in my FILs account. It was a meaningful amount of money but wasn't essential to pay for his care. I had made an estimate of the cost basis but he had acquired these shares at different times. I wouldn't have been real comfortable in an IRS audit so we just sat on the stock. He passed away and we sold right after transferring the shares to DWs and SILs Vanguard accounts. We sold immediately.