Hi,
If your mother gifts a share of the house to you, then she is subject to capital gains rules. If the property was her main residence for the entire period of ownership, then Private Residence Relief will cover any gain.
When you receive your share of the property and you are resident in it, then Private Residence Relief will apply to you for the period of ownership and the period that it was your main residence. You can see guidance on Private Residence Relief here:
HS283 Private Residence Relief (2024)
Thank you.