I've been thinking about family resemblance on TV a bit. In general, they tend to assume that the viewer will just accept that characters are related even if they don't look anything like one another. Probably in part because if the characters look too similar, the viewer might get them mixed up.
So, these guys are meant to be brothers:
Norman Reedus and Sean Patrick Flannery as Murphy and Connor MacManus (fraternal twin brothers) in
Boondock Saints. Note that they are always dressed very similarly and often have the same mannerisms/ choreographed reactions to sell the family connection. They don't look all that alike, but they're not markedly different, either - similar heights, same complexion, one has darker hair.
Meanwhile, TV turned around and cast Norman Reedus as Daryl Dixon, making these guys brothers:
Norman Reedus and Michael Rooker as Daryl and Merle Dixon on
The Walking Dead.
There's...less physical resemblance here.
And from
Bones, we have Seeley and Jared Booth, played by David Borneaz and Brendan Fehr
Interestingly enough, when that show wanted to give Temperance Brennan (Emily Deschanel) a sister, they chose Zooey Deschanel for the role - her real-life sister. However, when they wanted to cast this actress' father, they went with...
Yep, that's Billy Gibbons of ZZ Top as the father of Michaela Conlin. Apparently, you're supposed to think, 'just go with it.' [No, really, in real life her father is Irish-American, so it's not completely far-fetched. But still.]
Anyway, back to brothers....
Jensen Ackles and Jared Padalecki as Dean and Sam Winchester on
Supernatural. We'll need to do an older-brother-shorter-than-younger-brother for Fingon and Turgon, so that seems to have worked out here. Interestingly enough, the 'short' one here is 6'2". I would say these two don't look all that alike. I mean, they're both Hollywood-pretty, but beyond that......
Speaking of, here's their mom and dad:
Not exactly a strong father-son resemblance there!
Though not nearly as egregious as this:
Treat Williams as Matt Bomer's father on
White Collar. I don't see it.
My point in all of this is that while we might not want to pick people who are *radically* different in appearance, no strong family resemblance is needed to sell on audience on characters being family.
However, it can help to remind people of who is related and who is not....so there's no harm in planning out the families meticulously.