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.
All logical: he takes off the brunette, she takes off his pants, and they are each engaged in their usual business. The girl, by the way, will go far in the modeling business, where the ability to completely swallow a big cock, while smiling at it, is highly valued. Give a deep blowjob with a friendly face, this is memorable and the next time you choose a model, she will automatically be at the top of the list!
Let's try it.