Gertrude marries Claudius, the brother of her late husband Hamlet. Is this incest?
Here is a link from the University of Victoria comparing it to the biblical idea that man and woman become one flesh, thus the husband's brother becomes the wife's brother as well. This idea comes from Ephesians, and it was probably a very familiar verse to the people in Shakespeare's time.
"Incestuous sheets?" I agree, it's disturbing and I think it's incest. Comments?