Your capital gain is the difference between your cost basis and the selling price.
If the home was a gift, the cost basis is the price your brother originally paid in 2012, plus the cost of any permanent improvements that you or your brother or mother paid over the years (new roof, new furnace, remodel the kitchen, etc.). When your brother gave you half the home, he gave you half his cost basis. Then when your mother gave you her half, she gave you her cost basis, which was the other half of the brother's original cost basis. The fair market value is not involved.
The FMV would only be involved if you inherited the property. For example, if your mother gave you her half the house on the condition that she could continue to live there until she died, and then she did die, you might have an "implied life estate" and would be considered to inherit her half of the house. You would need to have the situation reviewed by a tax professional who can get the whole story.
If you did inherit your mother's half of the house, then your cost basis for the half you inherited is half the FMV on the date the other owner died, and your cost basis for the half your brother gave you is half his original purchase price.
