I am sorry for your loss.
Since you received a tax reporting form (1099-S), you need to report the transaction.
There will be no tax liability if you sold for less than the fair market value on the date of your mom's passing.
In fact, if the house was being held for investment purposes (i.e., no personal use), then you can actually recognize a loss on the sale (if the sales price less selling expenses is lower than the FMV on the date of death).
