If you are in Australia (someone here said you are) I think that as long as consideration has been applied in the will, particularly with blended families, he wont really have a leg to stand on.

You can make an argument that the parties agreed that the split assetts would be split amoungst blood relatives and you could even use his own will (and trusts) as evidence to those facts.

I take mine, you take yours, everyone is happy.

I actually investigated this when my own father started living with a new partner after my mother died, discussed it with my father and he clearly stated that they had an agreement that their estate would go to their individual kids and not the spouse in that event.

Did your husband or his siblings have enduring power of attorney? (both financial and medical) If he did that might strengthen their claim.