Capital gain incorrectly declared by executor?

Hello, I wonder if anyone can help with this query? When my ex husband died his will was out of date and still had me as beneficiary and his sister as executor. Naturally and rightly she inherited everything, apart from 20% of the net proceeds of the sale of his (formerly our) house, which he had specified in a separate letter was to come to me as part of our separation agreement in the event of his death or him selling the house, whichever came first. My ex sister in law acted as executor and, with the help of her accountant, made all the calculations and administered the estate, transferring the agreed 20% to me. She has now said that capital gains was somehow overlooked at the time and is now due on the difference between the probate valuation and the figure realised on the sale of the house and would I be willing to contribute 20%. I have said of course I will, but should it not be the case that we each receive £12,300 cgt allowance - she against her 80% and me against my 20% share, whereas her accountant has only factored in one allowance against the whole figure? I hope that makes sense and that someone can advise me. Many thanks

Hi Eleanor Morgan,
Where the property is willed to beneficiaries on the death of the owner and the beneficiaries later dispose of the property.
The beneficiaries must then work out if they have a capital gain from the disposal.
Each of the beneficiaries will have an annual exempt allowance.
Where the estate disposes of the property as the property, the estate only has one annual exempt allowance.
There is some guidance on Capital GainsTax: 
Capital Gains Tax rates and allowances
Thank you.