the Incest App. It's the winner of a contest sponsored by DeCode Genetics, which oversees the Icelandic genealogy database called Íslendingabók, or "Book of the Icelanders" (also the title of the first history of Iceland, written in about 1120 by Ari the Learned).
But there's another question to ask. Is sleeping with your cousin incest? It depends who (and when) you ask. Incest is a cultural phenomenon.
Sex with your father, mother, brother, or sister is uniformly considered incest--and wrong.
In Iceland, the Church's rules on incest were not enforced until the late 1100s, but by then they were even stricter. I give one example in my book Song of the Vikings, a biography of the chieftain and writer Snorri Sturluson.