OP please do take legal advice, but contesting a will, even as a spouse with a long marriage behind you, still makes it difficult, expensive and stressful. My mum spent 2 years contesting dad’s, legal fees ended up at about £50k, and ultimately, the house still had to be sold and half the proceeds given to OW. They did have a large house, so there was sufficient equity for it to be sold and still buy mum her own smaller house, outright.
Dad had left a relatively small sum to DSis and I to discharge his responsibilities to us - although we were older than your DD, in our 30’s with our own husbands, houses and careers so it was practically impossible to claim we were still dependent on him.
The law, only considers the legality of what he’s done, rather than trying to correct his poor moral judgement. Mum’s solicitor acknowledged he’d behaved appallingly, but his will had been correctly drawn up so it had to stand. He too died from cancer, very quickly (he died 2 weeks after being diagnosed with liver cancer) and suffered from depression. He had recently been taken off Prozac and this all happened quite suddenly after that was withdrawn. Mum tried the lack of capacity angle but it’s incredibly difficult to prove.
I’m really not trying to be negative, but as someone who has seen this situation play out first hand, just wanted to say how it worked out for our family. Dad died in 2008 and mum was left reeling for many years, it also affected how Dsis and I remember him. But she is now in her own lovely home, secure financially with his full pension being paid to her. And most importantly, she’s still alive enjoying her children and grandchildren, whilst he’s 6 foot under.