Dr. Drib, pdurrant -- I see Amazon's Family Library feature more as a vindication of the idea that it isn't intrinsically illegal to share your ebook library with family.
Why would it be legal to share with one family member and not the other?
Which means Family Library is an excellent way to easily share with one family member... but managing it yourself and sharing with two or three family members is equally valid.
Keeping in mind that someone who has DeDRMed their books has an obligation to be responsible about where they end up anyway.
And if the law said you can only share with one family member, I'd say it's just as silly as the one that says you can face criminal charges for DeDRMing for personal use in format-shifting.
And as pdurrant said, you can have attached children's accounts as well.
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Sometimes families share digital accounts.
I have never yet been arrested for letting my sister come into my bedroom and take one of my pbooks. My family often borrows pbooks from my cousins, and vice versa -- usually around Passover when we visit them. I don't expect to be arrested -- or indeed to be legally in the wrong -- if I let her/them take one of my ebooks as well.
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Now... my sister
happens to be connected to the same Kindle account as me.
But I sideload both our books --DeDRMed of course -- via calibre anyway. Most publisher title metadata sucks horribly.
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I absolutely would not even think of encouraging someone to "share" their ebooks with their friends. That is blatant piracy.