Firstly, unless the Will specifically states that one sibling will receive less, and if the estate is left to the two siblings, they will simply receive an equal amount. The gift will not be considered when dividing the estate unless the Will specifies this.
Secondly, IHT. Is the estate likely to incur IHT? There is a tax free allowance if £325 + an additional £175k if the main residence is left to direct descendants. So a total of £500k. This rises to £1m if the deceased has also accumulated the tax free allowance from a previously deceased spouse.
If the large gift + the remainder of the estate takes the total value over the tax free amount (less smaller amounts allowed as annual gifts) AND the gift was given less than 7 years ago there may be IHT to pay.
And none of that will affect the division of the estate as specified in the will. (The estate pays the IHT… which will in effect deplete the sum given to each beneficiary equally).
If the sibling who accepted the gift was so minded they could voluntarily even things out after the death of the parent either by a straight gift to the other sibling, or by doing a Deed of Variation to the Will, which is signing a document to say they agree for a portion of their sum to go direct to the other sibling.
Did the DP have a specific reason to make a gift to one sibling ? E.g they were seriously struggling to house themselves and DP grandchildren and also giving lots of care and support to DP, while other sibling is already wealthy and not able or willing to help on a regular basis?
People are allowed to make decisions with their money.