Hi Caroline,
The relevant instruction in the CG manual that covers this is:-
CG78310 - Foreign currency: assets acquired or sold for currency
I cannot find anything else in our guidance that mentions exactly what you have quoted, so I assume that is something external. The basic guidance when selling a property on gov.uk has a section on disposal of foreign property
Tax when you sell property and that just says any such gain is computed as normal unless the customer is non-dom, which you have not indicated you are, neither have you said it was at anytime your only or main residence.
So, as per the guidance at CG78310, the acquisition cost of each share in the property would be the sterling equivalent of the relevant currency at the time of acquisition.
For example, and just using the dates and figures quoted and assuming a 50% share each, the your mother's acquisition cost would be the sterling equivalent of 250/2 = 125k French Francs at her date of acquisition in 1989. Tour acquisition cost would be the sterling equivalent of 24k Euros at the date you inherited your share of the property in 2002.
The consideration for disposal for each of you would be the sterling equivalent of 42.5k euros each at the date of disposal in 2024.
Likewise, any incidental costs of acquisition or disposal and any enhancement expenditure that is still reflected in the value of the property on disposal would be computed at the relevant sterling equivalent rate of the foreign currency at the time of the expenditure.
Thank you.