The first thing that absolutely must be determined is who owned the home when it was sold and how was it titled? Just because it was refinanced with three people named on the mortgage does not mean that the other siblings might not still own a share of the home. (They might or might not have given up their ownership interest. You have to know this.)
Then, if you are specifically interested in the tax position of one particular son (sibling), where was the son’s primary residence over the past five years before the sale? A person can only have one primary residence at a time. Making many visits to see his parents or his mother, and making repairs to the property, does not make it his primary residence.
