We sold my mom's home which she purchased in 1997 but she added me to the deed in 2021. Both got 1099-S forms for half proceeds but how do I report on my taxes?

I know that my mother does not have to pay any capital gains on the sale as this was her primary residence. However, I never lived in this house and am confused on how I calculate the cost basis to determine capital gains for my tax return Do I use the market value of the house at the time that I was added to the deed?