My lovely DMIL was the executor of her DF's estate.
Her DH was estranged from one of their 5 children and the will divided the estate equally between DMIL and 3 of her siblings.
The sibling cut out of the will sent a solicitors letter challenging the will.
DMIL was open to dividing the estate 5 ways but the other 3 siblings didn't want to - not out of financial gain but rather to stand by their fathers wishes (I don't want to go into detail but the reason for the estrangement was very much down to the sibling and had been very acrimonious and had hurt their DF deeply - many attempts at reconciliation were made by him and rebuffed after which he changed his will). The siblings felt very uneasy about not respecting his wishes and the other sibling (who had remained estranged and refused to visit their dying father) choosing to only engage at a point of financial benefit.
In the end DMIL sent a solicitors letter in return saying they intended to honour the will given it was made when DF was of sound mind and there was no reason to challenge it (all expresses in legal terms).
Essentially it was saying sue us or shut up.
They did the latter as MIL's solicitors had predicted - simply as there were no grounds to challenge.
Not liking the will or thinking it's not fair isn't enough.
So based on that my suggestion would be to see a solicitor, lay out all the back story and if from what you've written you'll get confirmation that a challenge isn't likely to have any legs as it were.
Send a solicitors letter to that effect.
If they then get their own legal advice they will also be told that any legal action isn't worth it - especially given the sums you're talking about.
Alternatively you could try and appease them.
The estate vale is 245+60. So nephews 20k each your DH and sister 122k.
If you take the full value of the estate 305k and divide by 3 then your DH would get 101k as would his sister and the nephews 33k each.
The amounts here being pretty nominal.
However what I suspect they want is the 20k each plus a third of the house value so 47k.
As such I think your choices are to offer a 33k share or simply tell them to bog off via a solicitor.
I'd personally do the latter.