Peeking in through her cracked bedroom door, you get the best view of your sister. Really, you just casually glance in to see what she's doing - usually just coloring or doing homework. After some time coloring though, just as you lose interest, she starts to get comfortable.
Your cock jumps... you've been waiting to see this! She must get herself all worked up while she colors, something that she really enjoys doing. It gets her all relaxed... she's ready to play with herself, and you've got the best seat in the house to watch.