Something still does not seem right. How can it gifted to you, while your mother has a reverse mortgage on it. The title would have to still be in her name. Are you sure it was a gift while she was alive or an inheritance after her death? I don't care which one, but as Opus 17 said above, it makes a big tax difference. The amount owed on the home is not used in computing the gain or loss on the sale of the home.
