nadateturbe
In my opinion " genuinely doing the right thing" would have been ensuring your brother's children got his share of the inheritance.
In principle, yes.
But a person of sound mind makes a will according to their wishes regardless of what others think they should do with their money.
We don't know much about the circumstances of this family, only what the OP has told us, therefore it's impossible to know why the daughters of the brother didn't get the amount they expected. We don't even know what was in the brother's will..
Under 'normal' circumstances one would expect spouse and offspring to inherit after the death of the other spouse but for various reasons, that doesn't always happen. And in this case, we have no idea of the family dynamics or circumstances. So I think it's impossible to say whether the OP is being unreasonable or not.
If the two nieces think the grandfather was not of sound mind, or that he was coerced when making his will, then they can contest it on that basis.
The fact he didn't recognise them from photographs because they never visited him is neither here nor there. If he was of sound mind, he would at least be aware of their relationship to his son and, presumably, made his will on the basis of how he wanted the inheritance distributed.
Unless the OP gives more background information - and I doubt she'd want to do that - I think posing the question itself was rather pointless. Legally she has to do what the law requires her to do. Morally - well, that's a whole different ball-game and, as we don't know the family's background or situation, we can't possibly give any valid opinion.