It's actually a mix of other stats and as doc already pointed out, the higher the number the more likely the girl will rebel.
Increasing the following stats lowers rebellion.
Obedience
PC love
PC fear
Decreasing the following stats lowers rebellion.
PC Hate
Spirit
Confidence
The following stats fluctuate constantly. In general, if they fluctuate towards the undesirable end, they increase rebellion.
Happiness
Health
Tiredness.
The following traits also effect rebellion.
Broken will (-100 rebellion always)
Iron will (+high amount of rebellion. Can actually increase rebellion over 100. This is why it sometimes looks like torture has no effect on these girls. Also, obedience won't increase until I believe confidence and spirit are very low.)
Mind fucked (+50 obedience)
Finally, branding a girl a slave will drop rebellion by a flat amount. (forgot how much, think 20 or 40.)
I haven't played in a while though so this might not be 100% accurate. If anyone has an addition| correction, please post them.