As the other reply suggested, professional tax/legal help is advised. The way to do it (and it may now be too late) is for your mother to get all the proceeds of the sale, That way she can claim the home sale exclusion of the capital gain (up to $250,000), assuming she lived in the home at least 2 out of the last 5 years.
"Gift Tax" is somewhat of a misnomer. Even though a gift tax return may be required, very few people ever actually pay federal gift tax. The purpose of the gift tax return is usually only to document a reduction in the allowable estate tax exemption.
See https://turbotax.intuit.com/tax-tools/tax-tips/Tax-Planning-and-Checklists/The-Gift-Tax-Made-Simple/...
