We mucky up your mean family size whatsits but judging from other comments those are fairly mucked up in any case.

For the "surprisingly many of my friends are eldest children rather than not" does it mess with your analysis? It's a feature of the people you know that some of them are related to each other, if we ignore that we aren't modelling your friend space accurately. And yes, if you tend to know sibling groups you will tend to know a smaller proportion of exclusively eldest types. "Skewing" the maths by doing nothing to it in order to model a complexity for free is all good.