Hi,
Your aquistion cost will be your share of the probate value from your mother's will, plus the probate value of your share from your sister's will, plus the value of the share to buy out your brother.
You can claim private residence relief for the period that the property was your main home, have a look at helpsshett HS283 (
HS283 Private Residence Relief (2024)).
There is a capital gains calculator at
Tax when you sell property, which link in to registering, reporting and paying the capital gains tax online.
Thank you.