Joint tenancy to tenants in common residential home

My partner and his sister own a property (my partner lives in) for 17 years. Would like to swap to tenants in common so she has the smallest share as she hasn’t paid in , when they sell after being made tenants in common (10% share to 90% share) does either one pay capital gains tax ?

Hi,
If it has not been their main residence for the whole period of ownership, then yes. You can see guidance here:
Capital Gains Tax: what you pay it on, rates and allowances
Thank you.