By threatening her

'I swear to God, Olivia, if you don't get out of my clothes and room I'll fucking end you. You sick fuck.'

It wasn't quite the response she had been hoping for, but it gave her an idea. She walked back over to her own room, still dressed in his stuff, and she took a picture in front of her mirror to send to him. 'There. Happy now?'

It took a moment for him to reply, probably because he could predict where this was going. 'You're still in my clothes. Put them back.'

Olivia pulled the mesh shirt over her head, not that it was obscuring much of her breasts in the mirror selfie. Then she pulled the boxers to the floor, she could already see some of her juices accumulating inside of it. Stripped fully naked she looked at herself in the mirror and took another picture. Her breath stopped, heart pounding in her chest. Was she really going to send this picture? Did she really want her brother to see her like this?

It seemed like the safest way to experiment with her new found kink for showing her body. He wouldn't share her pictures with anyone. She hit sent before she could change her mind.