Partial truth

You have no idea how long she watched you snoop in her room, so you figure the best thing to do is just talk around her question. "I just thought I'd take a little look around your room. You and Tia are so close, I thought it'd be a good way to learn about you a little more. With you being away at school, I feel like you're the one family member I don't know that much about. I'm sorry I invaded your privacy."

"Awwww, that's actually kinda sweet." she says, walking towards you, her temper visibly cooling. "I'm sorry I snapped at you like that" she tells you, placing a hand on your arm. "I just wasn't expecting anyone here when I got home, especially in my room." Then, just as suddenly, her sarcasm returns. "So tell me, what did you learn about me looking through my underwear drawer?"

Just as you were thinking you might get away with this, Nicky squashes your hopes and your stomach drops. What lie could you possibly come up with to cover up this?

"I wonder what my sister would have to say about me finding you going through my underwear drawer... I can't imagine she'd be thrilled about hearing that, can you?"

"Nicky, I'm sorry. It was a bad idea and a mistake on my part." Time to tell the truth and live with the consequences you think to yourself. "To tell you the truth, I came over and had sex with your sister. I was still a little horny when she left for work and then I saw the picture of you two in your bikinis in the hallway. It got me curious to see what else you wore. I let my penis do my thinking for me. I promise, it'll never happen again. What can I do for you to keep this between us?" you ask her.

"Hmmmm" she says, as she ponders your question. "Keep this between just you and I? You know, Tia and I are very close. We tell each other almost everything. It'd have to be pretty big for me to keep this from her." she tells you.

"What do you have in mind? Just tell me what I can do for you."

"Well, like I said, we tell each other almost everything, and lately she's been telling me all about your thick cock and how good you fuck her. But as close as we are, we've never fucked the same guy before. I think I'd like to see for myself just how good you can fuck. I think if you can make me cum, I'll keep your little secret for you. What do you say?"

"Is she really offering this!? Is this some kind of trap!?" you say to yourself. There's really only two options to this proposal; fuck your girlfriend's sister or don't. You really care about Tia so you don't want to do anything to hurt her, but how many chances in life do you get to have sex with sisters? Besides, if you don't have sex with her, it's pretty much guaranteed that she's going to tell her sister, which would end up hurting Tia and likely ending your relationship. It seems like there's only one decision you can make.