The Guardian reports on the case of a pair of twins who had been separated at birth and adopted by different families. The twins later met and fell in love with each other and got married. After discovering they were brother and sister, they had their marriage dissolved by the high court.
This reminds me of a case I heard about somewhere about long-lost siblings falling in love and getting married. It apparently happens a lot, even in cases in which the siblings know they are related.
Talk about absence making the heart grow fonder.