Lucy Lume "Perving On Your Step-Sister"

Lucy is your step-sister and you have a habit of walking into her bedroom without knocking, usually to try and catch her off guard when she is getting changed but today you have walked in on her bent over in a short skirt looking through her drawers. You watch her climb onto her bed and while she looks under the bed she has her ass in the air showing off her ass and panties which gives you an instant boner! What will she do if she catches you perving on her?