Can a person with a pelvic fracture sit?

Yes: But ti depends upon the extent. Some pelvic fractures require bed treatment in others you are encourage to get up and move. It all depends upon severity, the force of trauma and many other fractures. There are very mild subtle fractures and sever life threatening fractures in the very large diverse category of "pelvic fractures".

Sitting: Yes a person with a pelvic fracture can sit up in bed or in a chair. If the pelvis fracture is shifted significantly from its original position it should be fixed so sitting is balanced.