[Grammar] The possessive form of noun

[Shamsiyan]

Martin Hewings has given the following example about the possessive form of nouns in the book “Advanced Grammar In Use”:

She's the boy on the left's sister.

I'm totally confused. Could you please explain about the meaning of this sentence?

 

[5jj]

I expect Martin Hewings explained it. She is the sister of the boy who is on the left.

 

[tzfujimino]

Hello.:-D
What an interesting construction! (It had confused me as well until I read 5jj's comments.)
I've just learned a new thing.
Is it natural English? (I guess so, since it is in the grammar book.)

 

[5jj]

It is certainly natural enough in informal speech.