His heart. That same one that was hammering in his chest so hard right now watching his sister's disheveled state. His cheeks were burning bright with embarrassment as well as a faint look of arousal. Still she was talking to him.

"I... it feels good. Really good. But it's still just my hand." He let that part slip before he could even think of stopping it. He was curious about sex. Above feeling it the way it should be. But his chances in that were slim to none.