How was the stock "inherited"? Was it a TOD account, joint account, et al.?
Regardless, your cost basis would be the fair market value on the date of death of your wife's mother, computed as the high plus the low on the date of death divided by two (e.g., 30 high plus 28 low = 58 divided by 2 = 29).
You would enter the sale in TurboTax in the Stocks, Bonds, Mutual Funds, Other subsection of the Investment Income section.
