Hi there-
While sitting after a Brazilian Butt Lift should be possible with appropriate modifications, the specific modifications necessary in your particular case will depend on where the fat grafts are placed, the period of time after surgery, and your surgeon's preferences.
I would not want any of my patients deviating from my advice based on what they read online, and I'm sure your surgeon would also be unhappy if he knew you were considering doing so...
As long as you have selected a Board Certified Plastic Surgeon who has a lot of experience, skill, and talent in the performance of the BBL, your best bet is going to be to follow his instructions and precautions carefully and completely.