Does any one know?
 

Does any one know how Bilbo can be Frodo's uncle if he was an only child?

I think this was something they added in the movie. I was rereading the first book today, and it says that Frodo is one of Bilbo's younger cousins:

If you have the newer paperback edition with the movie actors on the cover, you'll find that passage on the bottom of p.21

Well you see....

My mother, Primula Brandybuck, was Bolbo's first cousin (his mother and her mother were sisters). And my father, Drogo Baggins, was Bilbo's second cousin. Thus, I am Bilbo's first-and-second-cousin, once removed either way. However, he and I did use the terms 'uncle' and 'nephew' as terms of affection. [img]smilies/smile.gif[/img]

Ooohh, Frodo! Can I have your autograph? [img]smilies/biggrin.gif[/img]

One of my Best Friends, has a daughter, though i am no relation to her, i am her uncle.

What im trying to say is Uncle may be a title he just picked up, i mean he was realy the only family Frodo had, and even though he was probably more like a father it was more apropriate to call Bilbo an Uncle.