"...to leave thirds of her estate to each set of grandchildren"
Why would that be fairer in any way at all?
Suppose the grandmother leaves £90,000 and that there are three children and six grandchildren.
One child has three children, the next child has two children and the third child has only one child of their own.
If the grandmother leaves the money equally to the grandchildren then each grandchild will get £15,000. All perfectly fair.
Now, let's consider your idea of splitting the money by one third to each set of grandchildren.
Suppose the three grandchildren that are siblings are Amy, Ben and Charlie. The grandchild that is an only child is Timmy.
I can just imagine that conversation. "Well, sorry Amy, Ben and Charlie, but you only get to inherit £10,000 each from granny. Timmy gets some of your share and he will inherit £30,000 all by himself"
"Why is that? I don't really know why granny would give three times as much to Timmy as she gave to each of you. Perhaps she didn't love you as much as she loved Timmy?"
The grandmother has explained that:
"...has explained that as we are all settled and financially stable, she wants to help the younger ones who it will be harder for."
The grandmother has chosen to give some money to some people that are entirely unrelated to her rather than excluding them completely. There have been other threads on MN where the grandparent has chosen to exclude step-grandchildren entirely.
As other posters have said, these step children will also inherit through their biological father's family and their biological mother's family as well.