On a 7-9, she'll do it if you turn her on.

Jenna and Jessie look at you, still touching each other. Jenna is squeezing her little sister's breast while Jessie is moving her hand up your girlfriend's thigh and under her dress. "Let's see you strip for us first." Jenna says.
"Oohh, yeah," Jessie agrees. "I want to see your hot bod."
You strip out of your t-shirt and jeans, leaving your boxers on for now. The two sisters admire your body, and you move in so they can touch you. Jenna kisses you, running her hands all over your muscular shoulders. Little Jenna strokes your hard abs, while you squueze her tight teen ass. After a few minutes of the three of you touching and kissing each other, the sisters strip each other's dresses off. Both sisters have perfect, clear, pale skin, small perky breasts and hard little pink nipples. A crowd is gathering in the park, as they peel off each other's panties, revealing their smooth pink pussy slits. Both sisters lay down on the blanket, naked and ready for you to fuck them.