The Incest Bunch

[Forsaken Lover]

Both of Joff's siblings are perfectly sweet. Joff being a sociopath is just a thing that happened to him, not because he was born of incest.

Incest isn't that big a deal, anyway. The blood of the dragon do it because...well,they are the blood of the dragon. They're literally magic so they gotta keep that trout in the family.

[Raistlin]

Don't know where you got your source from but the one I found puts the chance of genetic defect or early death in a child born from first degree relatives (parent and child, brother and sister) at close to 50%.

Yeah, that's written by a psychologist who only cites a 1971 paper where many of the parents had pre-existing disabilities. Even assuming this one study, with its own confessed limited information, is generally accurate, it is useless for assessing any risks between healthy individuals.

[escobert]

The Targaryens were all about incest so it wasn't something new in the Kingdom.

[Ayen]

That, and it's socially acceptable in Westeros to mate with your first cousin. Tywin and his wife were cousins, so while not as close in blood as siblings would be, there's still more incest going on in the family than Cersei and Jaime.

[Mirage]

First cousins have a lot more genetic diversity than siblings, and second cousins way more than that again. In siblings, the only diversity you have is a slightly random mixing of the same two blueprints.

If you seriously are worried about the offspring, there's no need to be alarmed unless it's siblings and they're telling their kids to follow their example.

Of course, historically, it has been a lot harder to avoid the results of intercourse than it is today, so it's pretty understandable why it became a taboo. If it wasn't, I'm sure you could have gotten situations with two consecutive generations of solid inbreeding not too rarely. After all, if your parents did it, how bad could it be?