On a show my kids watch, iCarly, Carly and her brother really do have a sort of family-ish resemblance.
I always felt that the casting of Frances Conroy, Michael C Hall, Peter Krause and Lauren Ambrose as mother and children on Six Feet Under was totally bang-on. The resemblance between Conroy and Ambrose is uncanny.
Also, Lauren Graham and Alexis Bledel in Gilmore Girls very plausibly passed as mother and daughter, IMO.
The actresses playing Carmela & Meadow on “the Sopranoes” I always thought looked especially like mother & daughter.
I agree, but Sam and Dean look nothing alike. It’s like they didn’t even try when it came to casting. I keep waiting for the “Sam’s Adopted” or “Mary and the Milkman” episode. 
I thought the family in Family was believable.
I think perhaps their mannerisms and/or the way they carry themselves makes their relationship more plausible than purely their looks.
Julia Roberts and Kyra Sedgwick were cast as sisters in Something to Talk About. They do look like sisters even though the two actresses are unrelated.
No, they were trying for something else with the casting of that show. (As a friend of mine once observed,“Sam and Dean look nothing alike, unless being ridiculously good-looking counts as a physical feature”.)
FWIW, I’m never happy with the show’s mini-Deans. They’ve found the perfect little mini-Sam, though. That kid is adorable.
I think Debbie Reynolds was a great choice for Debra Messings’s mother on Will and Grace.
And, while it’s not the same, I thought Mayim Bialik was perfect playing Bette Midler’s character as a kid in Beaches; she not only looks like Bette, she had all her mannerisms down as well.
By the same token, I think Mila Kunis would be believable as a young version of Valerie Harper.
3rd for Frasier & Niles.
The Weasley family from the Harry Potter series.
I think it’s their mouths - Miranda Cosgrove has more in the way of cheeks than Jerry Trainor, but they both have similarly unusual smiles, which suggests a blood relation. (Probably got it from their mother - their grandfather from iWanna Stay With Spencer doesn’t have the same look to him.)
Yes, I watch iCarly regularly. It’s fun.
Really? Other than red hair, the siblings don’t look related at all. The twins have thin angular features and pointy noses (in profile), Ron has a full face with a wide, full-lipped mouth, and the oldest brother has curly hair, big ears, and looks, a little weird.
Ron looks like the dad and Ginny looks like the mom though.
Tobey McGuire and Jake Gyllenhal (sp?) from the movie Brothers.
Definitely, yes.
And Gwyn and Stacey as mother and daughter in Gavin and Stacey (second and fourth from left). Actually, looking at that I’m going to check that the actresses aren’t actually related and I’m not making a pillock of myself, because I think the resemblance is bang-on.
From NZ soap Shortland St these 3 are supposed to be sisters!
Its not so clear in this picture, but the South African actrees in the middle is of mixed race & originally didn’t sound all that kiwi!
& here is their red head TV mum http://images.google.co.nz/imgres?imgurl=http://farm4.static.flickr.com/3501/3888515775_342f84d6ab.jpg&imgrefurl=http://www.flickr.com/photos/reganrachel/3888515775/&usg=__-QV1Blf4XgEzKzspNibJ2nDy4KU=&h=500&w=333&sz=88&hl=en&start=5&um=1&itbs=1&tbnid=SJ-UQ834hYqVrM:&tbnh=130&tbnw=87&prev=/images%3Fq%3Dshortland%2Bst%2Bthe%2Bjeffries%2Bfamily%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26sa%3DN%26rls%3Dorg.mozilla:en-GB:official%26ndsp%3D21%26tbs%3Disch:1 & coarser features than any of the girls. The TV Dad has been written out - at a stretch you could picture him being the father of the tallest of the girls.
Was that old show with Michael J Fox & Meredith Baxter Birney Family Ties? The siblings looked nothing like each other or the parents. Same with Eight is Enough & the sisters on Petticoat Junction.
Kevin McKidd and Reed Diamond in Journeyman.
I know Nemesis never happened, but if Will Riker and Deanna Troi have a daughter, she would have to be played by Mila Kunis.
One episode of Psych (“There Might Be Blood”) introduced a guest star who bore some striking similarities to Chief Vick. It wasn’t long before I realized that she was going to turn out to be Vick’s sister – and she was.
Does actual family casting count? Casting Katherine Houghton as Katharine Hepburn’s daughter in Guess Who’s Coming to Dinner was genius. Ditto Sophie Thompson / Philidia Law as the dotty Miss and Mrs. Bates in Emma.
For non-related great casting, how about Sean Bean and David Wenham in The Two Towers / Return of the King?
Another one I just thought of is the three McPoyle’s on It’s Always Sunny. Even without all the makeup (and sweat, and robes) they still could pass as brothers and sister.
Long Riders.
Three Carradines, two Quaids, two Keaches, two Guests. Actual brothers cast as brothers.
.