Does any one know?

[Merri]

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:

But he had no close friends, until some of his younger cousins began to grow up.
The eldest of these, and Bilbo's favourite, was young Frodo Baggins

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