I've not been amongst the throngs of your critics, but it's OK to immediately back up and run from this position. Digging your heels in when espousing a concrete knowledge of how many guys your four sisters have had sex with, even if accurate, is nothing to promote, nor pat yourself on the back for, A. knowing in the first place, and B. using in an attempt to win a message board argument.
Though, while creeped out at the mention, I'm sure they appreciate the chaste nature with which you describe them. Tis a kindly gesture to their restraint on your part.
Or maybe they picked up on the "creepy brother who'll discuss our sex lives on a public message board with strangers" vibe and did their bumping of ... (how did you put it?) ... "Very Attractive" uglies at their partners' homes to keep out of your prying eyes?