Make one more rules change then ask to join them.

Josh decided to see if his sisters would let him join them, but before he did he pulled out his phone and went to the app and added 'Female orgasms are most intense during sex with one male and one or more other females with the pleasure increasing with the number of other women involved.'

He then walked into his sisters' room. "Hey there girls" he said to get their attention "Mind if I join you?"

Amy opened her eyes, which were dilated with pleasure and said "Mmm...I don't, and ... and I'm sure Elena won't ... I've heard it's better with a guy involved ... lets see if that's true."

As he quickly undressed he wondered why Amy had assumed Elena wouldn't mind. Then he 'remembered' that Elena spent more nights these days in his bed than her own and claimed she couldn't fall asleep without a cock inside her. He also remembered the night she first came to him and asked him to teach her how to be with a man. He had shown her the basics and, ever since, she had been coming into his room to practice every night before they went to sleep. Amy was right, she really wouldn't mind.

Once he was naked he walked over to the bed.

"O.k. bro" said Amy "you got a choice. My mouth or one of Elena's pretty holes. I'm sure you know which one you like best. So what'll it be?"