Are you in the UK or New Zealand?
We can only answer you based upon UK law.
Regardless, we are going to need some more information:
Who is saying this? Presumably the executors. Are they solicitors?
Presumably also she does not have the money to pay the tax.
Why do they not sell the property, deduct the tax and then simply sell the property?
In the UK, normally inheritance tax has to be paid by the end of the sixth month after the person died.
that can be problematical if there are no liquid assets but it is possible to come to an arrangement to pay in yearly instalments.
I think the problem may be that the executors will not be able to sell the house until such time as the tax has been paid which is why they may appear to being awkward whereas in actual fact, they are not really.
Can I clarify anything for you?
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The thread remains open for further exchanges.