There is a lot of confusion here and the family members are likely not to have dealt with the inheritance correctly, whether out of ignorance or arrogance I don't know.
The position at law is as follows. (I'm a solicitor for reference.)
FIL and W2 divorce and split assets 50/50. For the rest of her life W2 owns her 50% and FIL owns his 50%.
W2 then dies leaving her 50% of assets. Either she made a will leaving the assets to her two children in equal shares (I'm assuming this bit) or died intestate which would result on the same allocation. In neither scenario do the assets revert back to FIL unless the will says so (unlikely).
However W2 outlived one of her children (I hope I've got the timeline right). So whether will or intestacy, in the absence of a different express provision in the will, the surviving child gets her 50% and the late son's 50% passes to HIS children in equal shares. There is no scope for the executors or the other beneficiary to decide the children won't get their share.
If the children are not yet 18 (or other age specified in the will) then whoever is in possession of their share holds it "on trust" for them. Ideally a proper trust vehicle needs to be set up but if the executors failed to make those arrangements, the holder (trustee) of the funds holds them on trust for the children regardless.
It isn't clear what happened. It sounds as if the daughter was given 100% of the funds but didn't want to set up a formal trust vehicle for the children's share (unclear why and if the will didn't specify that this had to be done, perhaps because the will predated the son's unexpected death then the executors did not have to enforce one being set up) but the fact remains: whoever is holding the children's share holds it as trustee not as the beneficial owner.
So, the grandchildren go to live with FIL and he asks daughter for (some of?) The assets. She hands over 50%. This is actually correct provided that it is understood by FIL, daughter and grandchildren (if they are able to understand) that FIL is now the trustee.
Daughter is absolutely entitled to retain her own beneficial share and do with it as she sees fit. (She may or may not leave her assets to the grandchildren- no obligation.)
As trustee FIL can't do as he please with the funds he holds for the children. He CAN use it to fund the reasonable costs of their upbringing. But it would be much more transparent to do this within a formal trust vehicle so if he is challenged at any time he can prove all expenditure was wholly above board. He can't spend it on himself, save to discharge expenses he incurs on behalf of the children.
Did that cover everything?