@bobdrad wrote:
My understanding is that she can declare the home on her federal tax return as a primary residence.
How should we be declaring the home?
You don't "declare" a home or a residence on your federal tax return.
@bobdrad wrote:
Who can deduct mortgage interest for the new home?
You didn't say whose names are on the mortgage. As a general rule each borrower can deduct the interest that he or she actually paid to the lender. But someone who is not named as a borrower on the mortgage probably cannot deduct any of the interest, even if they pay it.
You can only deduct mortgage interest on two homes, your primary home and one other. For example, if you own another home, such as a vacation home, besides your own primary home and your daughter's new home, you can only deduct mortgage interest on one of the other homes, either the vacation home or your daughter's home.
Similarly for real estate taxes, each owner can deduct the real estate tax that he or she actually paid to the state or municipality. You can deduct real estate taxes on any properties that you own in the United States.