Degree of kinship and family names are two very different ways to define a family. For example Maud Windsor and Lord Culloden share the same degree of kinship (they are third cousins) with William and Zenouska Mowatt but do share the same surname as male line descendants of king George V.
So yes, according to the patriarchical system Xan and Maud belong to the same family but Zenouska and William don't (unless passing on the moyher's surname would keep you in that family) while Zenouska and Maud are far closer related as they are cousins once removed.
I don't think the queen mother would have been willing to say that her grandchildren by Margaret were not part of her family while her grandchildren by Elizabeth were...
All in all, it depends a lot on the perspective. For royal purposes it was much clearer in the past: a bride would go over to her husband's family but now the succession is gender neutral (or even when it was male preference, so women were not excluded) that would be much harder to argue.
Xan and Maud belong to the Windsor family; William belongs to the Mountbatten-Windsor family; Zenouska belongs to the Mowatt family. That is pretty straightforward to me.
Having more than one family on the balcony is OK though if it makes sense based on degree of Kinship. For example, Peter and Zara, although members of the Phillips family, should be on the balcony because they are the Queen’s grandchildren. Even when Charles is King, they should be invited too, as the children of the king’s sister. What I fail to see is the rationale to have third cousins on the balcony.
BTW, my examples above (re: Margrethe II, Carl Gustaf and Juan Carlos) were deliberate not only in terms of degree of kinship, but also because they all descend from Queen Victoria in maternal line , so they would not share the family name Saxe-Coburg and Gotha with Queen Elizabeth II ( if the Queen used it rather than Windsor of course).
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