Declaring and paying CGT on part of parents property

Hello. My parents were tenants-in-common. When my Dad passed in 2018, my Sister and I each inherited 25% of the house, with my Mum having the other 50% and staying in the house. The house was valued at £290k for my Dad’s probate. Effectively, each 1/4 my Sister and I had, was worth 290000/4 = £72500. My Mum died in December, and the remaining 50% of the house was passed equally to me and my Sister. We have just sold the house for £315k, receiving approx £311k after fees. I believe I need to report a capital gain on my quarter rising from £72500 to £78750 (£315k/4, ignoring the fees), so £6250. (I then deduct half the total fees from this figure, so liability down to about £4250). First question - when reporting this, do I refer to the value of the property as being the quarter that I have owned (Ie £72500, rising to £78750). I assume I would, otherwise the numbers don’t add up, and it calculates my CGT as four times higher. Second question - my Sister has lived in Australia since 1980. Is she liable for CGT also, and how would she 1) report this and 2) pay. (probably me doing it for her). Thanks.

Hi Erv2024,
Please have a look at Capital Gains Tax,  It contains a capital gains calculator, allowing you to save a copy of your calculation. Following on from the calculator, you can register for a capital gains account, so that you can report and pay the capital gains tax due.  When using this calculator, you would only show the values for the percentage that you own.  
Capital gains tax is payable by UK and non UK tax residents. As a non resident, your sister will need to submit a paper capital gains return form, which can be found at Report your Capital Gains Tax on UK property by post.  
Please note that the capital gains tax should be reported and paid within 60 days of the completion date.
Thank you.