I’m not a lawyer but I think it’s presumptuous to say he was going to sign the new Will on the day he died. What if he had a change of heart? There’s no way of knowing. Did these other people take care of him while he was sick? If they didn’t, perhaps he took notice of that. Did he ever tell his sister and niece of his plans to change his Will? At the end of the day, the people named in the legal Will are the rightful heirs. It will be on their conscience if they make a choice they know is wrong.
Stick with the old will when he was clearly in control of his faculties; if the beneficiaries want to give it to others that is up to them.