That the sister is interested in her foster brother's opinion is commendable. And assessing her worthiness from a man's point of view he can. But asking him to jerk off in front of her is kind of weird. He's going to get her, isn't he? Only this slutty girl is not scared at all - that's exactly what she wants. He ended up flushing a whole puddle on her tummy! Drove it.
Any girl knows what and how to make the other to excite, caress, what to get and bring to a maximum orgasm. A couple with sex toys teased each other, wielding their fingers, kissing and a lot of other interesting things.