This is the same situation my mum is in, when she married my dad he had left 90% of the marital home to his ex wife and children.
Both my parents financially supported them - my half siblings - well into adulthood. That's absolutely what my dad should have done but my mum had no obligation to spend her money on them. My dad was in his 80s and developed Alzheimer's before he stopped giving my half sister money every month.
And yet when he became ill they made very little effort with him. My half sister visited us from abroad and didn't even speak to him when she came over. Before she went home she posted all over Facebook how she was glad to spend that time with him 🙄 when he went into a care home, and later died, it was the same. My mum and I did everything.
They held a huge amount of resentment towards my mum and me (even though my mum was never the OW) and their mum encouraged this.
After dad died, my mum downsized and gave them several thousands each. They didn't even say thank you! She sold some of dad's old possessions and would have given them more if they had. Since she gave them money, they have made no effort to keep in touch with my mum or myself, all the effort has come from us, messages left unanswered.
Their mum still lives in the marital home which is now worth close to £1 million so when she dies they will inherit a substantial sum each. I am the sole beneficiary of my mum's will, and I will still inherit much less than they will, so no I have no intention of sharing that money with them. Why should I give money to people who have treated me with contempt and disdain my whole life, and didn't make any effort with our dying father? And most importantly, why should I when my dad made provision for them long ago, before I was even born?
If provision isn't made in the will for someone, there's usually a reason for that. Whether or not is "fair" is irrelevant. It's their money to choose what to do with.