George Michael and Maeby
George Michael Bluth first showed feelings for his cousin Maeby Fünke in the "Pilot" after she tried to annoy her mother by kissing him. The attempt failed (her mother took no notice), but it had a noticeable impact on George Michael. He became an ardent fan of the French film Les Cousins Dangereux (and the poor American remake Almost Cousins). He also begins a witch hunt to DNA test Maeby to see if she is really related to him.
In the season three finale "Development Arrested", George Michael tells his father, Michael Bluth, about his feelings. George tells him that him and Maeby are not cousins and that Maeby's mother is adopted, he continues to say that they should not hook up because they are still family.
Maeby and Steve Holt
Maeby Fünke exhibits interest in Steve Holt, who is also eventually revealed to be her cousin (fathered by G.O.B.). The two kiss in the first season when Maeby portrayed Benedick and Steve portrayed Beatrice in the school play Much Ado About Nothing. ("Bringing Up Buster") They dance at the school's Diversity Dance. ("Shock and Aww")
Maeby and Steve begin dating but don't take it to the next step. Instead, Maeby drugs Steve Holt with a forget-me-now pill. George Michael then informs her that Steve is their cousin. ("Forget-Me-Now")
Maeby and Annyong
When George Michael begins to show interest in Ann, Maeby tries to get his attention by kissing Annyong. Annyong is temporarily smitten by her, mimicking the relationship between George Michael and Maeby in "Pilot". ("Let 'Em Eat Cake")
Lindsay and Steve Holt
Lindsay displays interest in Steve Holt, unaware that Maeby is trying to woo him. Maeby convinces Steve that Lindsay is a transvestite and Lindsay backs off when she realizes Maeby is attracted to Steve. ("Sad Sack")
Michael and Lindsay
In the season three finale "Development Arrested" it is revealed that Lucille Bluth and George Bluth adopted Lindsay before Stan Sitwell — their business rival — could. With this new information, Lindsay tries to seduce Michael, but he persuades her out of it. G.O.B. then tries to seduce Lindsay but she denies him.
In the second season the episode "Afternoon Delight" plays on this theme when Michael Bluth and Maeby Fünke (and later George Michael Bluth and Lindsay) sing the Starland Vocal Band song "Afternoon Delight", being unaware of its sexual nature.
Oscar Bluth, George Bluth identical twin brother, also (very blatantly) hints that he is Buster Bluth's real father, through his relationship with Lucille Bluth. When Buster first learns of the truth, he exclaims, "My father is my uncle!"
Michael and Nellie
This theme is also explored in the episode "Family Ties" through the character of Nellie, whom Michael thinks for a time might be his older sister, but who herself presumes Michael is interested in her for sex. The fact that Nellie is played by Jason Bateman's real life sister Justine Bateman (who also starred in the 1980s sitcom Family Ties) lends the plot line an additional layer of comic complication.
Buster's oedipal complex
Another running joke throughout the series was Buster Bluth's Oedipal obsession with his mother. This strange relationship is highlighted in the episode "Motherboy XXX" where Buster says, "Whenever she'd change clothes, she'd make me wait on the balcony until zip-up—and yet anything goes at bath-time." Buster's mixed feelings about his mother are also shown through his relationship with his girlfriend and mother's nemesis, Lucille Austero.