If you (or your spouse) inherited the stocks from your father-in-law when he passed away, your basis is the market value on the date of his death, not the amount that he paid when he purchased them.
For inherited stock, all of the gains or losses are long-term, regardless of how long you or your father-in-law actually owned them.
So if you inherited the stocks, the date that your father-in-law purchased them, and the price that he paid, don't matter. You don't need to know that information.
