I was wondering if anyone knew the name of Hermans twin brother who appeared in many episodes and wore a plaid sports coat with a hat?

Herman had a twin brother named Charlie. He only appeared in one episode:

Season 1, ep. 9.
Knock on Wood, Here Comes Charlie (11/19/64)
Herman's evil twin brother, a con artist, comes to visit.

Herman also had a look-alike cousin, Johann. He, too, only appeared in one episode:

Season 2, ep. 64.
A Visit from Johann (3/17/66)
Herman's primitive look-alike cousin is mistaken for Herman by Lily.