Considering that the Westerosi had no issues with 300 years worth of mostly brother-sister pairings in their dragonly-appointed Valyrian rulers, I'd say that incest is socially acceptable enough within the ruling classes to make a marriage between cousins worthy of even a lifted eyebrow. Furthermore, even among the slightly-less-incestuous-but-still-cousin-marrying Lannisters, the real problem with the Cersei-Jaime match was the drama concerning Robert's heirs, not so much that Cersei and Jaime were doing it. It was scandalous, but mostly to honorable-ass Ned Stark. Everyone else was more interested in succession than they were grossed out by the fact that Cersei and Jamie were boning.
