She needs help preparing.

Your sister begins typing a response to Emma, then stops and deletes it, over and over. She wants to say yes, so badly, but something is keeping her back. Maybe it's buried shame at lusting after her sibling, she thinks. That would the therapist's answer, of course. But in her mind, she pictures herself bent over on your bed, sticking her shapely ass out for you, making an animal of herself. She grins and lets out a soft moan, imagining your thick meat forcing a way into her asshole. This isn't shame, she has nothing but desire for you and your body, damn the morals of it. She's afraid she won't be good enough at it.

Your sister knows from experience just how much cock you have to work with. It was an ordeal just fitting it inside her pussy, and she's used some very large dildos in the past while thinking of you. It just barely prepared her. She knows how well-trained Emma must be at taking you in her ass, and she doesn't want to come up wanting. So your sister decides to humble herself, and ask your girlfriend for help.

Yes, I so want to!

But... I need to get better at anal first.

Can you help me?

Back at your house, Emma smiles in the glow of her smartphone. With a few taps and swipes, she clears out her calendar for next week (mostly glory hole appointments and kink parties), and begins setting up a regimen for your sister to follow. Emma is excited to deliver you the ultimate anal experience, courtesy of your own slutty sister.

I'd be glad to. What's the biggest dildo you own?