She offers him something other then her pussy.

“Say Stevie. Want me to give you another blowjob?”
“We already did that yesterday.” Stevie said while he pulled down Emily’s panty’s. “I would really like experience your pussy again.”
Dammit, he was going for the one place she shouldn’t let him be.

“Wouldn’t you rather have my ass instead?”
Did Emily really just offer her butt to be used instead? She didn’t believe her own words.
“Maybe some other time, today I’m in the mood for your pussy.”

That’s when Emily felt her brothers cock head rubbing against her slit. Trying to find it’s way inside her.
“How about a boob-job then. Don’t you want to feel those big round breasts of mine on either side of your cock?”
This must have gotten Stevie's attention. He still her his cock head between her pussy-lips but he was no longer progressing his invasion.
“I’ll press them together really tight. It’ll feel wonderful.”