You and your family's best interests

There's been a leaflet sent round lately about the EU referendum happening in the UK. The government has sent this leaflet to all households, setting out the case for remaining in the European Union. Here's how it puts it:

The Government believes it is in you and your family's best interests that the UK remains in the European Union.

A friend mentioned on facebook that he was disappointed that despite the £9m spent on this leaflet, it contained this grammatical error. I begged to differ: this is stylistic variation, not a mistake.

He argued that it should be your and your family's best interests, as both conjuncts should be possessive. This is right, as you should be able to leave either out and it still be grammatical.

But there's two complicating factors here. The first is that the possessive your is kind of already a combination of you plus 's, so perhaps the 's is redundant. I don't actually think this is the case, because I think there's another reason why it's OK to say you. 

It's to do with the nature of 's. This is what we call a clitic, which means that it's phonologically dependent (has to attach to) another word, but is not as tightly linked to the word as a suffix like the plural s. While the plural suffix can only attach to countable nouns, the possessive can attach to a much wider range of things. The only requirement is that the noun it refers to be within the phrase it attaches to. This means that we can have phrases like the following, where the possessive attaches to something other than the possessor, and sometimes not even a noun:

The woman with the long hair's dog
The guy I was talking to's friend
The girl dressed in blue's mother

It's a little more complicated with coordination, of course, as we have to have the 's referring to both conjuncts. But I think that's OK. Language Log have discussed this before, and examples like this are all right:

I and my friend's work (a bit clumsy in my opinion, but not bad)
Me and my friend's work (perfectly well-formed)

So, unusually, I'm with the government on this one, both in terms of their grammar in in staying in the EU.