Reluctantly

"It's kinda personal... " You start.

"Hey!" She protests "I, a virgin, just offered you sex and now you're too chicken to say what fantasies you have? " Lexi laughs "Come on, tell me who and how or I'll die from curiosity."

"Uh... now after I've seen Justine's tits..." You blush quite heavily "I'm kinda attracted to a titfuck idea and I'm gonna imagine just that, her caressing my dick with her boobs. They're ideal for that and..."

"... and after that she would suck you off?" She finishes for you

"Yes, yes... now please go to your friends. They're waiting" You practically beg, sharing your little fantasy with your virgin sister has made you even hornier.

"Sure my big-boobs-loving brother, sure." Lexi smiles in a way that implies she has an idea. Some nasty idea, but you fail to understand what she has in mind as your mind is not too clear right now.

She leaves you alone for a nice jerk-off session, you don't limit your fantasy to Justine alone, Diddy, and Melanie are there too, but those D-cups are the main picture before your eyes for the whole time. If you only could get them in your hands...

____
A few hours later you hear a soft knock on the door, after closing your 'World of Tanks' session you open the door to see Justine wearing her sexy cheerleader uniform.

"I'll be short, pervert. I owe your sister a favor and she wants me to repay it in a rather unpleasant way. You may do whatever you want with my tits and I'll blow you. But no undressing me below my waist, it's not going to happen. Understood?" She's clearly mad. She doesn't want you. But what matters is she is determined to let you enjoy her as a present from Lexi. That must be some serious favor she owes her.