What's with incest in Game of Thrones?

Marrying your cousin is acceptable in some of these elite families. Royals often married a cousin of some sort although usually not a first cousin. It even happens in living memory in the real world. Franklin Roosevelt married his IIRC third cousin, Elanor Roosevelt.

The Targaryens get a pass on incest because they were royalty and were foreigners, descended from the blood of Old Valyria. The fact that their incest was tolerated doesn't mean that everyone else had license to boink their sisters. And we see in the Dunk and Egg tales a scene where Dunk reacts with revulsion to how casually Aegon Targaryen discusses the possibility of marrying his sisters. So the incest taboo in Westeros clearly survived the conquest by the Targaryens.

To add to what Allan said in the books the Targaryens were said to be descendants of dragons and dragons had to keep their bloodline pure in order to retain their ancestry. It was often alluded to that many of the Targaryens' children were weak or stillborn. (as they had few offspring)

And with Craster he was a wilding which are generally referred to as being "uncivilized" or "barbaric" which is why, even though it was completely wrong for him to marry his own daughters no one felt like this wa out of line for "Someone who was even an outcast from the wildings".

It has been a while since I read the books but I think most of that info is mostly correct.

Cousin marriage is marriage between people with a common grandparent or other more distant ancestor. In various cultures and legal jurisdictions, such marriages range from being considered ideal and actively encouraged, to being uncommon but still legal, to being seen as incest and legally prohibited.

"Game of Thrones" is trying to help Liberals promote their next (sick) movement. After being successful in making gay marriage legal in many states in the USA, Liberals are now doing their best to promote incestuous marriage in America.

Consensual sibling incest seems to be rising in popularity, so it was put into "Game of Thrones".