Paradigm shifts.

Look at it this way: in Europe, during the time when monarchies had absolute power, incest was the best solution for "keeping blood pure" so that the prince(ss) didn't have to marry someone the royal family didn't like. When the best example of this is the Habsburg dynasty, which produced a Spanish king said to have "died of poison 200 years before he was born" because of how hideously deformed he was, there's going to be a negative view of making babies with your sibling/first cousin in a day and age where this isn't necessary anymore.

In the US, primarily in Appalachia, incest was more or less a necessity due to the society being more isolated than the rest of the country. There are still plenty of states in the US where marrying your first cousin is legal. Though I'm not so well-read on it, cousin-cousin marriages were the most common; culturally, the idea of "kin" is pretty much from the concept that "this person is related to me, but I don't know to what degree", because in any community during this time, everyone is probably a cousin at least twice removed.

Since we now have a better understanding of how in-breeding leads to various defects, of course it would be frowned upon, because most people understand that even if a sister can be a good wife, a nephew probably isn't a good son.