Ask your sister for help in completing the quest (nicely)

You're an idiot, so thinking things through takes you a little longer than most.
While you were pondering your predicament, you've been steadily facefucking your teary eyed little sister, ignoring her muffled protests as you drag her mouth up and down your gradually expanding member.

She's been begging you to stop facefucking her for a while (at least as far as you can make out. It's hard to understand someone when they're trying to talk with their mouth occupied) but you've been paying little attention to her, your hands and dick on autopilot as you continue to violate her face enthusiastically.

You pull your dick out for a moment and look down at Vanessa, wondering exactly how you should phrase your question so you can trick her into helping you complete your quest.

"Hey Vanessa, I got a question for you."
"Pu- Please, stop. Stop this. I won't tell anyone. Mmmph--- gurk---"

You reflexively stick your cock back in your sister's mouth when she asks you to stop and start fucking her face again. There's just something about the years of ingrained inferiority complex that makes you want to do the exact opposite of whatever your sister wants. And unfortunately for her, right now that means that every time she asks you to stop **** her, it just makes your cock get harder and makes you want to pound her face even more.

"Shut up. Your older brother is talking to you, and you're going to be respectful for once and listen when I'm telling you something. And stop crying. I'm not going to stop **** you. Think of it as payback for all the years of back-talk and sass I had to take from you, smart-ass."

"MMmmph! COUGGFF"

Being the idiot you are, you forgot what you were doing and focus on the mouth **** at hand for a few more minutes while you try to gather your thoughts again.
Right, Quest. Smart-ass that's **** on your cock here could help with that. And on an unrelated note... she has a smart mouth which has been being appropriately disciplined for the years of **** but that still does leave two other holes that need to be addressed.