I don't know how old Maggie's younger sister is supposed to be. If Maggie was supposed to have been college age, I take it that her sister was at least 15-16 when we met her, and will be the "audience approved" legal age as the show progresses if she survives being eaten by walkers.
That being said, "legal age of consent" in Georgia is 16, but yeah, you are correct in what the show would allow for a cross-state audience where most viewers live in an 18-and-up environment.
Next, though, true to the characters, Darryl might be interested for "a man's physical need," but this girl who's not a natural fighter / survivor would not really be his type. That's what attracted Darryl to the growing strength shown by Carol, and also having more in common with Carol's more country simpleton background (OK - I was going to say white trash) versus Herschel Greene's family members (yes country) but more formal, religious, background.
The only way it would happen is if the younger Greene sister decided she wanted to seduce Darryl. He's got his own code of honor it seems and would protect her (maybe even reluctantly since she's not up to his caliber as a fighter) but would not initiate.
If the youngest Greene survives for longer on the show (and she's not that interesting IMO - not compared to Maggie for sure!), then maybe.
It would return us to the inverse of when Rick comes back to find Shane with Laurie.
Instead, Carol returns to find Beth with Darryl.
Not as interesting I think - but maybe since Carol has taken it upon herself to take "Shane steps" to protect the group from those that were sick and would turn.
She has to have a reckoning with Tyrese first.
But finally, if the Zombie Appocalypse were realistically portrayed to its greatest extent, people would hook up because their partners would be "the last person on earth...." LOL