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Take a look at the notes here and here.
Gifts made in the seven years before the death of the donor are included in the deceased estate for Inheritance Tax purposes. The nil-rate band is first used against gifts made in the seven years before death in chronological order so only £250,000 of the £900,000 gift to your elder sister would have been chargeable to IHT. If you elder sister has paid all the IHT including that on the property left to you and younger sister, then she has already done you a favour. Normally, where there is not enough cash to settle the IHT liability, assets are liquidated to pay it and everybody will contribute based on what is left to them regardless of the lifetime gift having priority access to the nil-rate band . Beneficiaries are only liable to the extent of the IHT on what is left to them. I'm afraid that you will need to consult a family lawyer to discuss options open to you including suing the accountant.
I hope this clarifies your position but let me know if you have any further questions.
The nil-rate band can be split by agreement but you have to recognise that the gift was made before the death of the donor and, is therefore entitled to have priority access to the nil-rate band. The recipient of the pre-death gift has no IHT liability beyond the liability on that gift. The accountant was correct in allocating the nil-rate band to the £900,000 gift as you will read here. If your sister paid the IHT on the £400,000 left to you and your younger sister too, then she has clearly exceeded her responsibility as far as IHT is concerned already.