Double Genitive: Twice as Much Possession

Alice e-mailed me with a wonderfully tricky question:

I recently heard someone say: She is a friend of my sister's and I. This doesn't seem right. Should it be: She is a friend of my sister's and mine? Or something else? Thanks for your help with this!

This is an awesome example of how easily we can get our knickers in a twist when there are too many things going on at once. In this case, there is a compound pronoun plus something nasty called a double genitive. Many English teachers and brainiacs would get this wrong, but Alice got it right. Bravo, Alice, you smart thing!

What the heck is a double genitive? Think of it as possessive overkill. In the sentence She is a friend of my sister's, possession is signaled twice: first with the preposition 'of' and then with the apostrophe + s. It sounds terribly awkward, but the grammar is correct.

Double genitive = preposition 'of' + possessive noun or pronoun. He's a friend of my boss's. Mr. Smith is a colleague of Melanie's. She's a favorite hairdresser of ours.

Where does it all go wrong? Even people who get the double genitive right in a sentence with a single pronoun will often get confused when faced with a compound pronoun.

Let's break it down:

1. Write the sentence twice, using simple nouns instead of the compound structure.
   She is a friend of my sister's. She is a friend of I.
   a. The first sentence is a correct use of the double genitive.
   b. The second sentence is incorrect because it includes a subject pronoun instead of a possessive pronoun.
2. Replace the subject pronoun with a possessive pronoun.
   She is a friend of my sister's. She is a friend of mine.  That works!
3. Then put the conjunction back together.
   She is a friend of my sister's and mine.

Correction: She is a friend of my sister's and mine.

More examples of the double genitive: 

• She's a former teacher of mine. She's a former teacher of Sam's. >> She's a former teacher of Sam's and mine. Or: She's a former teacher of ours. 
• He's a neighbor of my sister's. He's a neighbor of my brother-in-law's. >> He's a neighbor of my sister's and brother-in-law's.

This may look and sound dreadful, but it's correct.