Extension to property paid by one sibling - Added value VS sibling costs?

[ColinK]

The estate is to be shared by three siblings.
This question only relates to the property - not any other asset.
The will declared that he property should be split 3 ways. All siblings agreed that Sibling 1 should buy out the other two siblings share upon the death of elderly parent.
Sibling 1 lived with parent and paid £100k for an extension to the property, for her use, before the death of the parent - the will was not changed, no Deed of Variation has been created. Sibling 2 and 3 were not particularly happy with building the extension but did not want to "rock the boat".
The probate office have accepted that the extension forms part of the estate.
Sibling 2 and 3 do not seek to benefit form the value of the extension.
The property has been professionally valued and it is accepted that the extension adds £50K to the value of the property.
Sibling 1 expects the house value to be shared among the three siblings is - current value less £100k
Siblings 2 and 3 feel the house value to be shared among the three siblings is - current value less £50k
What is a court likely to decide if required to intervene?

This question is not considering the long term family consequences of such a dispute and is not seeking comments on this (important) element.

[atticus]

Of course the extension to the deceased's house forms part of the deceased's estate.

On this information no one can predict what a court will rule. What evidence is there of any agreement between Sibling 1 and the parents around the building of the extension? If the court determines that Sibling 1 has a claim it may express its ruling as a percentage of the sale proceeds and not a fixed number.