Mom and Dad gifted 50% of the house to their two adult sons in 2012, proper gift tax return was filed in 2012, the house was purchased in 1989. Only one child lives in the house as his principal residence, the other child has his own separate principal residence, so I believe the mom and dad and one child will each qualify for the 250,000 exemption, the other child may have to pay capital gain tax, the sale proceeds will not be "gifted" back.
