IHT only becomes relevant if the father's estate plus this gift is over £500,000. If this is not the case then the entire discussion around IHT is irrelevant.
"When you apply for probate you need to declare this money and will probably have to pay the 40% inheritance tax."
It appears that the OP may be the executor of the will (she said "my brother is suggesting that I go against the Will").
As executor, the OP will need to declare the gift to HMRC.
However, as beneficiary of the gift, the OP will not have to pay any inheritance tax.
Instead, the value of the gift reduces the amount of the nil-rate band (currently £325,000) available to the estate.
This will mean that there is more IHT to pay by the estate before it can be distributed to the children according to the will.
So, practically speaking, the OP as beneficiary does not pay any IHT on the gift.
If the estate is already over £500k (after the gift) then the previous gift of £89k will have the effect of increasing the amount of IHT payable by £35,600.
Assuming that the will divides the assets equally, this means that each child will receive £17,800 less from the estate.