Go in.

Any more waiting and you feel that you'll burst with excitement.

You walk to her bathroom door hurriedly, and inch open the door. Once it's fully open, you peek inside, not hearing anything different.

You see that she's still showering, as her shower curtains are drawn. You can make out he faint outline of your slim sister's sensual body as she washes herself the running water.

You could go and pull apart the curtains now, or wait for her to finish and do it herself.