Thank you for clarifying.
You are correct that no UK inheritace tax is payable by your non-UK domiciled mother on a non-UK situate property.
For UK tax purposes you are treated as receiving the property at it's "market value" when your mother died. The market value of the property will be the price which that asset might reasonably be expected to fetch on a sale in the open market. Since the property was not exposed to the open market when you inherited it, it can be tricky to determine (and not the same as the probate value if that was understated).
Since you will be gifting the property to a connected person (i.e. your sister) you are treated as selling it at (again) it's open market value. So to summarise, you will be subject to UK capital gains tax on the increase in market value from Dec 2018 to Sept 2019. There are no exemption savailable to you, but I hope you will find that the increase in market value should mean there is not much CGT to pay.
Also bear in mind currency fluctuations. The market values need to be calculated in GBP as at Dec 2018 and Sept 2019 so the gain may be more or less than you expect.
The property will also be subject to stamp duty land tax if your sister is buying it from you for consideration greater than £125,000, as apposed to a gift. I assume there is no mortgage on the property.
Please let me know if you have any further questions?
Kind regards, Peter