"Sit upright" could be used either to indicate moving from lying down to sitting up, or to adjust the way you're sitting so that you're not slouching. "Sit up straight" is more of a command you give to someone who is sitting lazily, leaning over against the wall, or slouching when you want them to pulltheir shoulders back and look straight ahead.

Sit up! Sit up straight! Sit upright... the first two are likely to be used in Commands! Exclamations! The third I think is the same meaning but less likely to be BARKED at you