Ask her not to

He decided to go visit Stephany in her room and ask her to keep what they do a secret.
He found her laying on her bed rubbing her clit and using her phone camera to take pictures of what she was doing.
He walked over and knelt between her legs and went down on her bringing her off with an orgasm.
Her then took the camera and erased the pictures of him going down on her.
"Do you have any idea how much trouble we both could be in if anybody saw me doing that to my sister?" He flicked through the rest of the pictures in her phone.
She took pictures of his cum on her face and she quickly pointed out there was no way anybody could tell it was his.
Then he found pictures of Stephany licking a cunt.
She refused to tell him who's pussy she was eating.
He quickly found several photos of her eating the same cunt.
Seeing her face covered in pussy juices he wondered if his face looked the same after going down on Stephany