@prh47bridge is right. Unless s33 of the Wills Act is expressly excluded or unless a contrary intention is obvious in the Will. Therein is the issue, you can't know what your ExDMIL's Will says, unless she has shared the content with you and, even then, if she has capacity she can update her Will at any time with no moral or legal obligation to inform anyone of the change.that's if she has a Will!!!!
I'm a regular MN poster but have name changed. I am a private client lawyer and this is my area of practice.
To be honest I've been thinking about this thread today and I just had to comment.
OP, no one can begrudge you wanting to secure your daughter's future. Especially in light of the fact that her father hasn't provided for her as he should have done through there childhood.
However, and it is a big however, your daughter (or anyone else for that matter) is not ENTITLED to receive any inheritance from your ExDMIL, or anyone else! It is entirely her money to do with as she pleases. If she does leave her estate to her son(s) then, again, it is their responsibility to ensure adequate provision has been made for their dependents. This would mean it's your ex's responsibility to make a Will to ensure his assets pass to his child(ren), if that's what he wants to do.
As other posters have said, if your ex died intestate (without a Will) then the Intestacy Rules applicable (at the time of his death) will apply to who receives his estate and in what shares. If your ex did marry then, under current law, his new spouse would receive the first £270k of his estate and all his chattels (personal items) and 50% of what remains. The other 50% of what remains goes to children of the deceased and, if more than one on equal shares. If he isn't married then it all goes to his children (at 18 - held on trust if they are minors). That's a very quick overview if he doesn't have a Will.
That said it is your ex's responsibility and YOUR responsibility to provide for your daughters future. Not her grandmother. What she does with her money is really non of your business. I'm sorry but it isn't. I see it all the time at work. We even get family members calling in trying to arrange a Will for their parents / grandparents. It's so entitled and wrong. Believe me the lawyers taking those kind of calls think it is so inappropriate and dare I say, grabby. Fair enough helping your old dear arrange her Will that she wants to do but no, you can't be in the meeting and it isn't your decision as to who she leaves her money to! Rant over!
Teach your daughter how to make her own way in this world, provide for her yourself if you want to in your Will, discuss with your Ex his provision for her as her other parent but that's where it would end. No one is entitled to revive an inheritance. If anyone gets one then how lovely, what a lovely gift to revive, but never bank on it as an easy solution to money issues.
That's just my thoughts. Completely not interested in getting into a debate. We all have our own opinions and that's why MN is so interesting and you asked a question in the first place!