Thank you for your question...
gain from sale of residential property is chargeable to CGT.. There are reliefs given against the gain before arriving at chargeable gain.
As your sister owned 1/3 share of the property, she would claim private residence relief against her share of the gain for all the period (since her name was added to the deeds) she lived in the house as her main residence. She would also get relief for final 18 months of ownership as it was her main residence at some point during the period of ownership.
To answer your question, she may have to pay some CGT depending on the amount of gain (her share) and relief available as aforementioned. One would only know what thsi is once the calculations have be performed.
Once the chargeable gain is calculated, she would get gains allowance against it and the balance would be taxed at 18%, 28% or a combination of both depending on her taxable income in the year of sale.
More information of private residence relief can be found here
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.