From a plot stand point it is.
Well, in what manner? And, why is that an issue?
Anyway, I have a general concept for how this whole thing would work. It would start out with Rider as a servant/slave at the Matou household (not particularly well-treated, in general). She wouldn't like Shinji or Zouken, but would be friends with Sakura, because Sakura is nice to her (she'd also notice that something was not-quite-right about Sakura's general treatment). She would also be generally opposed to the Roman system (and, in particular, slavery), although not to the point of openly fighting against Nero (not that she could do so anyway).
However, the Matous would find that she was getting close to discovering something they don't want her to find out, and so would plant evidence of her planning to assassinate Nero. She'd be arrested and taken away to be brutally tortured and then executed, and Shinji/Zouken would consider her gone. However, Sakura would feel bad for her friend, and would send evidence to prove her innocence to Nero anonymously. This would result in Nero working out that she had been framed (which hadn't been Sakura's intention), and Zouken would force Sakura to admit her guilt, which she would agree to do to protect Shinji.
Nero would then free Rider, and offer her the chance to punish her fake accuser, and then retain her as a slave. Rider, who would be too angry at Sakura's betrayal to think clearly about it, gladly accepts, and Sakura is taken, in chains, to Nero's torture dungeon, where Rider would await her. Rider would then start torturing her very harshly, talking about betrayal etc., and gaining lots of pleasure from torturing someone who had caused her so much suffering. After a while, though, she'd start to wonder
why Sakura had betrayed her like that, and would start questioning her (under torture) about her motivations and how she'd done it. Of course, she would get non-sensical answers and, eventually, Sakura would break and admit she wasn't responsible. Rider isn't sure if this is true, but still feels guilty that she might have tortured an innocent friend, and starts questioning her more gently and kindly, and investigating herself. Eventually, she's convinced that Sakura is telling the truth, but she doesn't have enough evidence to bring down a powerful family. So, she instead takes Sakura as her slave (freeing her from the Matous), and offers to let Sakura punish her. Sakura refuses, not wanting to perpetuate the cycle of revenge and knowing Rider had suffered already, but they gradually develop into a sexual relationship, with plenty of BDSM involved.
As for Zouken and Shinji, I expect they'd get their comeuppance at the hands of Rin, when she realises what they put her sister through....
As for rider's torturer I have no idea
Yeah....
Well, I'd think a guy would make sense, because that makes it different from Sakura's torture, and allows her to be raped.
We need to do a bit of world-building here, I think, to work out who will take what roles. In particular we need someone who would be liable to support Nero and also inclined towards being a torturer....
