Tender response, then order her to leave

You pull her to your feet, your hard-on nestling between the bunched folds of her dress, and place a hand on the side of her face, stroking her cheek.

"My dear sister," the shock is still evident on her face, "you must realise that you were in the wrong. I do not hold it against you, but please try and control yourself in future around me. We will continue your training soon, and we'll make a proper lady of you. I've plenty of things to teach you, so that you can be the best you possible and make a Lord a truly happy home and wife in the future. Now, go about your day and I'll see you at dinner. If I've the time, I'll look for your lost things."

She tentatively nods, and stumbles towards the door, straightening her dress and flattening down the fabric as she leaves your bedchamber.