Apparently Pennsylvania Gov. Tom Corbett needs an education on the difference between siblings and spouses. Since I have both a husband and a sister, here is an explanation of how a sister is not like a husband.
The only reason for the remark is to use the structure of legal analysis -- but not the substance -- to produce an "ick!" reaction in the listener. In short, gay marriage freaks Gov. Corbett out just as much as incest. It's a big joke at our expense.