Is this a homework exercise or a test question? This TurboTax community is not a homework help site. We're here to help people handle their own taxes. Where does MeeshkaDiane fit into the scenario that you described? What is your relationship to Joe, Peter, and Mary?
A gift is not a "buyout." In your scenario, no one is buying anything. Or did you omit some crucial details? Joe and Peter (not Mary) will have to file gift tax returns if the value of their gifted shares is more than $17,000 (for 2023).
To determine the effect on Mary's basis you need to know whether the estate used an alternate valuation date, the fair market value of the home on either the date of death or the alternate valuation date, and the fair market value on the date of each gift. You need dollar values, not just fractions of ownership.
There is no capital gain until someone sells something.
