Stepdaughter's Naughty Habit: Watching Her Masturbate in the Hallway

My stepdaughter's been up to something naughty, and I've caught her in the act. She's always in the hallway, fingers deep in her pussy, tits bouncing, and eyes locked on the mirror. I can't help but watch, my cock twitching as I imagine what she's thinking. One day, I couldn't resist, and I grabbed her hips, whispering in her ear, "I know what you've been doing, and I want to help you with that."