Yes, give her a compliment

"I think that is obvious, isn't it? " You smile nastily "After your crazy offer I'll imagine that I accepted and enjoy one fantasy about wild **** with my hot, naughty sister" You don't know why you are so honest now, most likely because there is little to hide after offers like the one Lexi made a minute ago "She is way hotter than her friends."

Lexi giggled once again "How can you be so perverted and so sweet at the same time? And what would you do with that mysterious sister?"

"Lexi, are you starting that again?" You ask "Please, just let me jerk off, alone."

"Hey, I'm just curious, I'm a girl after all and I'd like to know what guys like in me. As for your boner, you can start with me around, I don't mind. In fact.... " Lexi smiles devilishly and removes her shirt to reveal her bra covered tits. "I'll help you and provide a nice sight to make jerking off more fun."

You quickly close your eyes " Lexi, we shouldn't... This is horribly inappropriate. You should be so sexual towards me..."

"Oh come on" She sighs. "If that is so sexual, then millions of magazine models get **** every day. What will happen if you'll look at my body while pumping your boner? Nothing!"

You reluctantly open your eyes and say a short "wow", as Lexi removes her bra leaving her now bare firm melons of flesh inches from your face.

"I like that you like them" Lexi smiles once again and judging by her red face and erect nipples she is obviously as aroused by the situation as yourself. "Now take off your pants, enjoy the sight and say aloud what you're imagining!"