Sean Astin is the son of Patty “But they’re cousins!” Duke and her second husband, Michael Tell. He was adopted by her third husband John “Gomez” Astin but grew up thinking his biological father was Desi Arnaz, Jr. (with whom his mother had an affair around the time of his conception). Patty Duke insists still that her marriage to Michael Tell was never consummated and that Sean could not have resulted from this union, but DNA testing says otherwise. Sean now has a relationship with all three men (Astin, Arnaz & Tell), but regards only Astin as his “father”. His real-life daughter appeared as on-screen daughter in the last LOTR movie.
…is the son of Michigan football great Tommy Harmon.
New York news anchor Felicia Taylor is the daughter of actor Rod Taylor.
Actress Anne Baxter’s uncle was architect Frank Lloyd Wright.
Actors Peter Graves and James Arness (are/were) brothers. Likewise game show MCs Jack Narz and Tom Kennedy.
Francis Capra, who plays Weevil on Veronica Mars, is the son of Frank Capra. The grandson and great-grandson, too, seeing as how he’s Francis Capra IV.
Then there’s Liv Tyler, who played the daughter of Elrond, is in real life the daughter of Steve Tyler of Aerosmith. When Steve asked her if she’d like him to sing at her wedding, she said no.
I don’t blame her.
Denise Crosby (Tasha Yar in STNG) is the granddaughter of Bing Crosby.
Gates MacFadden (Dr. Crusher in STNG) used to be a muppeteer, which isn’t a family relation, but I think it’s really cool.
Liv Tyler didn’t know Steve was her father until she was in her teens, right?
Yep. According to the IMDB bio, she grew up with Todd Rundgren (who had married her mom). Steve kept bringing his other daughter over to visit and Liv noticed that she and Mia were practically twins… her mom then fessed up to what happened.
Frank the actor is the grandson of Frank the director.
Joely’s mother is Connie Stevens. She and Carrie are half sisters.
Whoops! Forgot about Connie. (Imagine that…)
Tony Goldwyn (Ghost, Joshua, Last Samurai) is the grandson of MGM co-founder Sam Goldwyn. Ironically considering the source of his family’s fortune, his family was furious when he became an actor.
Sam Robards is the son of Jason Robards with Lauren Bacall.
Geraldine Chaplin is the daughter of Charlie Chaplin, granddaughter of Eugene O’Neill and great-granddaughter of James O’Neill.
The children of Natasha Richardson and Liam Neeson have one of the most distinguished as well as one of the most generally screwed up ancestries in show business. Their parents are famous actors, their aunt Joely Richardson is distinguished, their grandmother is Natasha Redgrave, their maternal grandfather is Tony Richardson (who was gay [or bi]), their great-aunt is Lynn Redgrave (quite the odd family story there), their great-grandfather was Michael Redgrave (who was gay/bi) and their great grandmother is Rachel Kempson.
Actor Simon Harrison is a grandson of Rex Harrison.
Kate Beckinsale’s dad, Richard, was a popular TV actor in England. He was in such series as Rising Damp. He died of a heart attack at 31.
Nicolas Cage is the nephew of director Francis Ford Coppola. Nick changed his name from Coppola to Cage because he wanted to succeed on his own merit.
Cathy Silvers, who played Jenny Piccalo on “Happy Days”, is Phil Silvers daughter.
Kiefer Sutherland (of 24 fame) is the son of Donald Sutherland and Shirley Douglas (who, incidentally, was the daughter of Tommy Douglas, former Saskatchewan premier and an actor himself).
Steve Tyler (Aerosmith) had a daughter named Mia with his first wife, Cyrinda Foxe. Mia is a plus-sized model and is married to Dave Buckner, the drummer for Papa Roach. Steve’s other daughter is of course Liv Tyler, who’s mom is Playmate Bebe Buell. Liv married former-Spacehog musician Royston Langdon, and they just had a son, Milo, a few days ago.
John and Michelle Philips of the Mamas and the Papas had a daughter named Chynna, who was herself a successful singer in the group Wilson Philips. Chynna is married to William Baldwin, one of Alec Baldwin’s brothers, thusly making herself the former sister-in-law of Kim Basinger. Oh, and Michelle Philips was briefly married to Dennis Hopper.
Frank Zappa is almost as famous for the bizarre names of his offspring as for his music. His eldest child, Moon Unit Zappa, is married to Matchbox Twenty drummer Paul Doucette, and their daughter, Matilda Plum, was born a few days ago (hey, maybe Matilda and Milo will grow up and marry each other!). Moon Unit’s brother Ahmet recently married actress Selma Blair (Hellboy). Her other brother, Dweezil, was the longtime partner of singer Lisa Loeb.
Heather Locklear is the ex-wife of Motley Crue drummer Tommy Lee (also ex-Mr. Pamela Anderson) and the current wife of Bon Jovi’s Richie Sambora. Tommy’s bandmate Nikki Sixx is married to Donna D’Errico, also of Baywatch. Eddie Van Halen is/was? married to actresss Valerie Bertinelli, with whom he has a son.
Tracey Reiner, “Betty Spaghetti” in A League of Their Own, is the daughter of two distinguished showbiz families, father Rob Reiner/grandfather Carl Reiner, and mother Penny Marshall/producer Garry Marshall.
Jack Cassidy had two Teen Beat sons, David and Shaun, the latter with Shirley Jones.
Kirk Douglas has spawned two generations of actors of varying degrees of importance.
Campbell Scott is the son of George C. Scott and Colleen Dewhurst. His wife, Kyra Sedgwick, is the niece of Warhol protege Edie Sedgwick.
Isn’t Kyra Sedgwick married to Kevin Bacon, or am I thinking of someone else? 
To clarify, Garry is Penny’s brother. Also, Carl Reiner’s wife is
Actually, Kyra Sedgwick is married to Kevin Bacon. According to the IMDb, Campbell Scott is divorced from a woman named Anne. Campbell does have two show-biz siblings – brother Alexander is a writer and stage manager, while sister Devon appeared on The Tony Randall Show and other sitcoms.
Mississippienne: Wolfgang Van Halen’s parents are still married.
The late Audrey Meadows (Alice on The Honeymooners series) was the sister of Jayne Meadows, known for being Mrs. (now the widow of) Steve Allen as well as appearing as a panelist on many game shows.
Sheila MacRae, who played Alice on “Honeymooners” sketches that were incorporated into The Jackie Gleason Show, was the wife of Gordon MacRae, who was perhaps best known for his role in the film version of Oklahoma!. Their daughters Heather (Clarissa Explains It All) and Meredith (Petticoat Junction) became actresses. Sheila and Heather are still alive.
Speaking of Petticoat Junction, creator/writer Paul Henning’s daughter Linda played the role of Betty Jo, but was originally billed as “Linda Kaye” to avoid the appearance of nepotism. In later seasons, she was credited as “Linda Kaye Henning”. From 1968-73, Linda was married in real life to Mike Minor, who also played her husband “at the Junction”.
Oops, meant to say that Carl Reiner’s wife, Estelle, is famous for saying “I’ll have what she’s having!” in the film When Harry Met Sally.
Alan Hale Sr. of Little John fame from TheRobin Hood* flick starring Errol Flynn was the father to Alan Hale Jr., better known as The Skipper in Gilligans Island.
If you ever see them, they were almost identical.