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Under law, a parent is under no obligation to leave anything to their children. In the scenario where a child is left 0%, the child would only be entitled to make a claim against their parent's Estate for inadequate provision if they had been financially dependent on their said parent.
Likewise, a Grandparent does not have to leave anything to their grandchildren. Normally, clients would not leave anything to the grandchildren and leave their Estate equally between their children, on the basis that the grandchildren will in due course inherit from their respective parent.
On the basis that your Wife's sister has died, it would be normal and reasonable for her Mother to leave 50% to your Wife and 50% to her late sister's children.
I hope this assists and answers your question.