Insist you all watch porn together; it's not really that incestuous.
From Create Your Own Story
"I don't mind," you shrug, pretending that watching porn with two hot girls doesn't phase you at all. "Let's go up."
"Yaaaay!" Shayla grins, skipping upstairs with Adam close on her heels. Kara is significantly less enthused, so you stay to walk with her at her pace.
"I can't believe we're doing this," she sighs, sounding bitter.
"I wouldn't worry about Adam, it's-"
"I don't care about watching porn with my brother. We're actually pretty comfortable with each other. I care that Shayla has this huge obvious crush on Adam and now I have to watch them get all turned on together."
"I'm sorry, Kara. Is this going to be okay?"
"Yeah. But it would especially be okay if you sat with me... maybe we could fool around a little if they start to... I mean, just for show to piss her off. She likes you, too, and she can't have you AND my brother!" Kara's blushing like crazy as she rants and plots. Of course you love the idea of fooling around, even it she says it's just to bother Shayla. But is the information that Shayla wants your dick too tempting?