So the amount being split, after payment of IHT, is £510,000 ish (I have taken £40-£45k for each of 12 grandchildren as £42,500 each)
If you hadn't been given this gift, it would have been £560,000 and each person's share would have been £46,667 - a difference of £4,167.
IHT is 40% of everything over £325k so pre-IHT it would have been £716,667 ish. Overall, approx 22% of the whole estate has gone to IHT.
The other beneficiaries have no right to expect DH to refund the gift you&he were given, and it is quite right to refuse this, but I do think it would be reasonable for your DH to fund the share of the IHT bill which is due to your gift have been made within 7 years of the death. That would be £10,930.
So my advice would be for your DH to volunteer to receive only £31,570 as his share of the estate, allowing the other 11 to receive £43,493 each. That is an appropriate gesture IMO. Otherwise you effectively have his 11 cousins each chipping in £910 to cover the IHT on the gift that you & DH received, which doesn't seem quite fair.