Who is Frodo Baggins' uncle?

The Longfather Trees of the Hobbits show Frodo with five Uncles:

Dinodas Brandybuck

Didonas Brandybuck

Saradas Brandybuck

Rorimac (old Rory) Brandybuck

Dudo Baggins

Frodo's father, Drogo Baggins was Bilbo's second cousin, so they were 2nd cousins once removed. However, in the book Frodo is commonly referred to as Bilbo's nephew, so Bilbo is referred to as Frodo's Uncle. Baggins.