Hello and welcome to the site. Thank you for your question.Technically, you should have reported the gain on every occasion you transferred a percentage of your share of the property to your sister unless the agreement was that property would be transferred on receipt of the last instalment and that date is now.If you have lived in the family home all the time as your main residence (from when it was transferred to you on the demise of your father) then you would be able to claim private residence relief against your share of the gain.If on the other hand, this is not your main residence and it is a second home/investment property, then the gain would be chargeable to CGT. You would get gains annual allowance against it and the balance would be chargeable to CGT at 18%, 28% or a combination of both depending on your total income in the year of sale.Here are some calculations assuming the whole gain is chargeable to CGT
Due date of payment of CGT is outlined under Key deadlines of "What to report to HMRC for Capital gains Tax" here
I hope this is helpful and answers your question.
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