I like this line you seem to be going towards of them having to do ever more daring stuff to reach the mark of making him twice as large.

I wonder how far you'll make them go.

I also love that this suggests he gets much more easily excited by his sister than by 2 beautiful, but less familiar girls. I wonder if this implies that when he strangles her and started to fuck her under the pretense of "I'm curious and she's out so she won't mind", it was really that he's fantasizing about fucking his sis and doesn't want to admit his incestuous kink yet.