It is also worth remembering that the twins didn't target Rowan just because he would be useful to their plans, but also for revenge. I imagine that they would like to capture or kill all of the heroes. Rowan was just the most vulnerable and the only one they could reach with their limited power at the time. In fact I fully expect the twins will be directly targeting the other heroes in future chapters.
Anyways, my point is: this is why they are always twisting the knife in Rowan, forcing him to watch their cruelty or forcing him to choose between to two painful choices. They are casually cruel to everyone, but for Rowan they go out of their way. The point is to make him suffer. The question then is what is the end point of their revenge. For Andras I imagine once he's had all the fun he can have he would just as soon kill Rowan. Jezera on the other hand would probably rather break him and keep him as a pet, though I could see even her killing him in frustration if he refuses to break for too long.
The main thing that keeps their revenge against him in check is his usefulness to them. His usefulness even lets him push back to a small degree. However if they manage to capture other heroes, and manage to recruit other followers capable of managing the castle and troops, then Rowan's usefulness will wane, and will allow them to ramp up their cruelty towards him.