Oh brother

I just saw "Seed of Deception" (13 April 1958) on Starz Encore Westerns on August 8, 2018, where Bart is shot and later asks if Bret is thinking of avenging "his only brother", thus suggesting that Bret and Bart are the only two sons of Beau "Pappy" Maverick.

James Garner as Bret Maverick left the show between the 3rd and 4th season, and was replaced by Roger Moore as their cousin Beauregard Maverick, possibly the son of their uncle Bentley Maverick, for the 4th Season, but he left the series during the 4th season.

Robert Colbert appeared as third brother, Brent Maverick during two episodes of the 4th season, but didn't appear in the 5th season.

Wikepedia's episode list:

https://en.wikipedia.org/wiki/List_of_Maverick_episodes

Says that in "Pappy" (13 September 1959) there is another indication Bret and Bart have no brother:

"...Also note that at one point, Pappy explicitly mentions that he raised two children and didn't want a third; this would seem to contradict the later appearances of Brent Maverick, the third Maverick brother."

So I can't help wondering what was the explanation for the existence of a third brother and if there was some story explaining away the evidence against that third brother.