] The "incestuous, adulterate beast" seems to me to be saying that Claudius is those things because he lusted after Gertrude and then married his "sister" (in the sense that his brother's wife is his sister).

That would explain the "incestuous" bit of course, but that's not the same thing as adultery. Or could the Ghost just be adding that to Claudius' list of crimes, knowing he has committed adultery at some time in the past with some one?