My deepest condolences, BToldboy on the loss of your DW (dear wife). I understand your wanting to move on with someone who "seems to make the grief less," and don't think it's anyone's business to object, including your SDs. However, I think it's 'normal" for them to fear you're forgetting their mum, and I agree with the poster who said you should reassure them.
Also, grief, as you may know, has several stages. As you go through each stage, you may feel differently about your younger friend and may treat her differently. That, in itself, is a reason to take things slowly.
I take it you and she have been good friends for years, so I doubt she's just being a golddigger. However, one can't be sure, Imo. I don't generally have a problem with large age differences if both people are adults. But I still feel one needs to proceed cautiously if the gap is wide. And I definitely think this is true in your case.
Timing is not your strong suit, I'm afraid, given your 11th hour marriage that wills mentioned. Another reason to be careful, Imo.
There seems to be an assumption that your SDs are in your will. Are they? If so, I would resist any effort to influence you to change it. And if you want them to get the house, eventually, it might be a good idea to give it to them now, with the understanding (in writing) that you get to live there for as long as you live (which I hope will be a very long time).
Also. people. I want to express my sympathies for everyone here who has become widowed or has lost a parent, as well as for those who have been or saw a relative being fleeced.