I've performed hundreds of autopsies and not once did anyone sit up. I've asked everyone I know who has worked in pathology and they have also never witnessed this. I had one of the pathologists I worked with explain things to me and the easiest way I can explain it forward is soon after we die, there is still some "electricity" in us. This electricity can cause small movements, like twitches, but there is no sitting up happening. If someone told you that they were pulling your chain. I've witnessed the small twitches on organ donors who were still being ventilated, but they were still being perfused. I'm pretty sure rigor doesn't set in for hours after death and then only lasts for up to 12-14 hours.
As I'm typing this, I realize it's been 12 years since I've done an autopsy. Doesn't seem like it can be that long.