Her twin brother

The first thing Jenny noticed about her twin brother John was his raging erection.

"Whoa there, bubba! What's got you so hard?" she giggled.

"Huh? What, you mean this? Was your bath too hot or something, sis? You know it's always been like this. Are you feeling alright?"

Looking back at some of the family photos, she realized that every male in them is also sporting an erection. Huh, that must be a side effect of my wish to have sex be possible anywhere at any time. I guess that always having an erection would make having sex on the spot a lot easier.

Upon closer inspection, she also noticed that all the females in the photos have fluids dripping down their thighs and just then realized that she too is dripping wet and not from her bath. Oh, this is gonna be fun! she thought to herself.

Out loud she said "Yeah, I don't know what came over me just now. Probably just need to get fucked."

"Well, let me screw your head on straight, then." he said with a wink.

"Oh, ha ha. You're so punny! Anyway, let's go downstairs and watch a movie or something and you can just pound away at me. Or were you hungry and came up here looking for someone to eat out?"