She is very angry

Any pity Sansa had for her brother was surpassed by the anger she felt toward him.

"I cannot believe you did this!" She scolded, still covering herself from his prying eyes, "You should be ashamed of yourself!"

"I am," Bran said, feeling defeated and embarrassed. He just wanted this to end. However, he couldn't stop admiring his gorgeous sister, so his hard-on persisted. Sansa noticed and was horrified.

"Were you jerking off to me?!" Sansa gasped. Brann dropped his head in shame. "You were! Oh my god I'm your sister. I could just kill you right now!" She needed to let out her rage, so she brought her hand back and slapped her brother in the face. Brann recoiled from the blow, not expecting it. The slap stung, as he had strong feeling for his sister, and didn't want her angry with him.

"Is there anything I can do to make up for it?" Bran asked, almost to tears.

Sansa considers this for a moment, thinking of an apt punishment. Part of her was still horny, and she knew she could use Bran in any way she wanted. She could also tell her father, and surely get Bran in trouble.