I remember an episode of Taxi where Tony went through a whole episode trying to adopt a boy. He adopts the child in the end, but the boy is never seen or mentioned again. Louie's brother also appeared once and I don't think he was ever mentioned again.

The Simpsons has also had close family members of several characters appear once or twice, then disappear without much explanation. Some big ones are Jessica Lovejoy, Herb Powell, Krusty's daughter Sophie, and Frink's wife and baby.

Barney's cousin Virgil - The Andy Griffith Show

cHUCK ON HAPPY DAYS HEE WAS RICHIES BROTHER ONLY THREE OR FOUR EPSIDOES
GEEENY PICKOLOW I HEARD SHE WASS MEANTION AS JONNS FRIEND. ONLY SEEN HER ONCE ORTWICE. I THINK SHE WAS IN SOME OF THE OLDER EPSIDOES.

jUDY ON FAMILY MATTERS. THEY USED TO HAVE HER ON SHOW WHEN IT FIRST STARTED THEN SHE WASN'T THEIR ANY MORE

BRADON ON STEP BY STEP HE WAS IN THE EARLY EPSIDOESOF STEP BY STEP BUT NEVER MENATION WHAT HAPPND TO HIM LATER ON .

Harriet and Rachel's father appeared in an episode of Family Matters and was never mentioned again.

Rhoda Morganstern during her run on Mary Tyler Moore had another sister who appeared in a fourth-season episode. She was never mentioned again. She did not even appear in the spinoff series Rhoda.

The kid Dorothy's son Michael had with the black woman from The Golden Girls.

On Family Guy Lois had a sister who had a baby in one early episode and was never seen again.

I could be wrong but on Friends Rachel had a few sisters, (each played by some big name guest starlet) who would make an individual appearance during a sweeps period and would never be heard from again.

I believe Shirley's brother made only one appearance on an early episode of Laverne And Shirley.

I could be wrong about the last two.

Three's Company - Janet's sister appeared in an episode but never mentioned again, not even at Janet's wedding in the finale.

All in The Family/The Jeffersons - Henry Jefferson, George's brother, was seen on All in The Family for a few episodes and disappears. We never see or hear from him on The Jeffersons.

Lois also had a Brother and was Never Mentioned againas well.

The Brady Bunch - Carol's parents were at the wedding in the pilot but were never seen or mentioned again after that. (also the girls' cat, Fluffy. :lol: )