My mother died in 2012. Her house already had my and my sister's name on it. We sold it. I received a 1099S with my portion. How is that treated in regards to taxes?

The difference between the proceeds you received for the sale (as shown on the 1099-S) and your cost basis is considered to be a capital gain. Your cost basis is your portion of the market value of the home as of the date of death of your mother (or the alternate evaluaton date 6 months later, if chosen by the executor of the estate). Since the house sold quickly it's doubtful that this gain is very much, if anything at all, especially taking into account the cost of the sale such as real estate agent's fees. You should report the sale on Schedule D, using the cost basis as described above. If the gain is positive you will owe capital gains tax on the gain. But if it's negative you cannot take a write-off, as losses on the sale of personal property are not deductible. So if it's a loss you report $0 gain, and no income tax is due on the sale.