Howard Sprague is from his very appearance portrayed as the quintessential only-child mama's boy with a domineering mother. Prior to the arrival of her offspring, there has never been any indication that Howard had a sister. In fact, the entire mother/son dynamic at play in the earliest appearances by Howard in which his mother goes to such extreme lengths to keep the leash of those apron springs as free from slack as possible strongly argue against even the possibility of a sibling. Of course, it is never made entirely clear exactly what the circumstances of the absence of Howard's father is so it is theoretically possible, if highly unlikely, that Spud is the seed of a half-sister borne not by Howard's mother, but another women entirely. At any rate, the sudden appearances in Mayberry of Howard's nephew and Goober's niece represent just two of a very long list of interruptions in the continuity of characters.
Brad David that plays Howard Sprague's nephew, Spud, was at the time married to Nicole Jaffe. Nicole Jaffe was best known as the original voice of 'Velma Dinkley' from the Scooby-Doo series.
It's surprising that strait laced and buttoned up yet, experimental Howard didn't sympathize with his nephew's free spirited view of the world and need to express himself. In the season 8, episode 15 of TAGS, Howard quit his job and moved to the Caribbean to become a beach bum. In season 3, episode 2 of Mayberry RFD, Howard took a Sensitivity Training course and ditched his gray suits for more toned down clothes and sandals.