Yes, extended sitting can negatively affect brazilian butt lift results
In order for the fat injected during the butt augmentation procedure to produce long-lasting results, the fat has to be aspirated properly, processed properly, and injected properly. And the post operative period is just as important as the surgery itself.
I tell all my brazilian butt lift patients that they should not sit at all for up to 3 weeks after surgery, though most patients can return to work in 2 weeks. I would strongly discourage a 9 hour flight 12 days after surgery since...