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.
Well why only in the mouth, and in the butt too. I think it would be a good idea to fuck such an attractive woman in the anus as well. I think she would be very happy with that too, you can see how much she loves sex in any form.