Stephenson has negative trade value, and Sullinger and Bradley individually are valuable enough that they shouldn't be traded except for a 1st round pick.
So basically, the Celts are giving up way too much in your proposed trade.
I think Bradley + #16 straight up for Stephenson + #9 makes a lot more sense. Even that might be too much to give up.
Stephenson is basically a more talented Evan Turner with a lower floor but a higher ceiling. I think he could work in Brad's system, but he and Turner don't make sense on the same team, so they'd have to dump ET.