I almost forgot the infamous Kickboxing Academy in Chyler Leigh’s romantic interest is played by her brother Christopher Lee.
I didn’t learn until today that Michael C. Hall is married to Jennifer Carpenter, his on-screen sister in Dexter. They met on the show.
Ramon Estevez was also in that. He played one of the stockade guards.
Barbara Hale played Della street, secretary to Perry Mason, on the TV show and movies starring Raymond Burr. In nine of the Perry Mason TV movies, between 1985 and 1988, her son, William Katt, who was famous as the Greatest American Hero and as the Sundance Kid from Butch and Sundance: The Early Days, played Paul Drake, Jr., the son of the original Paul Drake from the TV series.
Just a minor nit-pick…Luke played Ritchie and Owen played Eli.
For those who don’t know him that’s John Putch who also played Bob on One Day At A Time, the tuba playing, shy boy, who secretly lusted after Barbara.
David Tennant’s dad had a cameo in Doctor Who, as a footman in the Agatha Christie episode.
Fredric March and his wife Florence Eldridge made six movies together; in three they played man and wife. In the 1936 Mary, Queen of Scotland, he played Bothwell, husband of Mary, while Eldridge played Elizabeth Tudor; in the 1949 Christopher Columbus, he played the title role to Eldridge’s Queen Isabella. And in the 1935 version of Les misérables, he played Jean Valjean to Eldridge’s Fantine.
But in Scrooged, BDM played Bill’s father.
Tom Hanks and Rita Wilson also appear in That Thing You Do!
And his Brother John played his brother. But his brother Joel played a house guest of john who was not related. So Joel holds the distinction of appearing with three of his brothers in a film while being the only of the four siblings who was not cast as a relative.
Dom DeLuise and his sons all appeared on “Stargate”, but none of their characters were related.
Olin has also recently appeared on “Brothers & Sisters” (which he produces and occasionally directs). Wettig is a regular cast member.
William H. Macy guested in an arc on “Sports Night” which starred his wife, Felicity Huffman.
Idina Menzel, the wife of Taye Diggs, recently guested on Taye’s show “Private Practice” but her character had a romantic interest in Tim Daly. When Tim Daly was starring on “Wings” his sister Tyne had a guest appearance as a woman who was not related at to him at all.
On Battlestar Galactica, Edward James Olmos stars as Commander Adama, while his son Bodie Olmos has a bit recurring part as rookie pilot Hotdog.
Skirting the OP with a couple of twins… 
Linda Hamilton’s twin sister Leslie appears in Terminator 2. Once as Sarah (in a flash back) but also once as the liquid terminator pretending to be Sarah.
Nicholas Brendon’s twin brother Kelly Donovan appeared as the other half of Xander’s split personality in a Buffy: the Vampire Slayer episode.
Susan St. James appeared on the Drew Carey show as Oswald Harvey’s mother. Her niece Christa Miller played Kate.
Brother and sister Christopher and Charlotte d’Amboise played in Andrew Lloyd Webber’s Song & Dance, and did a pas de deux together that was most decidedly unsibling-like.
Twins Don & Dan Stanton played a security guard & the T-1000 version of the guard in Terminator 2 as well.
Twins Deidre and Andrea Hall appeared on Days of our LIves, first as twin sisters Marlena and Samatha Evans. After the latter was killed, the actress reappeared as Hattie, a diner waitress who knew everybody’s secrets.
The Nanny had a plot where Fran Fine was casted as a dreadful Juliet, in a production designed to lose money for tax reasons. Her then husband Peter Marc Jacobsen played Romeo, leading to the following dialogue between him and another female actor, both dressed in Elizabeth garb:
Actor: How did she get casted? She is horrible.
Jacobsen: She must have slept with the producer.
Actor: I slept with the producer and all I got was two lines
Jacobsen, very smugly: I got Romeo.
Dan Aykroyd and Donna Dixon in “Spies Like Us” and “Dr. Detroit”.