Josef Mengele, is that you?
Full disclosure I work in oncology research (clinical trials), so this is close to my heart. The sestras are autonomous beings, so yes, I would still be on their side even if they somehow could cure cancer (not that cancer is one disease or this would ever be realistically possible). Any possible medical gains that could come of holding them, monitoring them, etc against their will are completely irrelevant. Medical research involving human beings is governed by some basic principles, including that consent to participate must be freely given, the rights/welfare of the people participating in the research take precedence over anything else, and that research will be overseen by independent ethical review boards. We have internationally accepted documents that guide us when doing medical research. In the US we also have federal regulations and further protections in place for vulnerable populations, like children, inmates, those with special needs, etc. Granted the show takes place in Canada, but Canada likely has its own laws pertaining to research, and would be on par with the US.
The Tuskegee syphilis experiment is one of the best known instances of "doctors" unethically experimenting on people, but it's unfortunately not the only instance. Usually when this happens, it's to a vulnerable population, like incarcerated people or those living in poverty. Looking at OB in the context of the real world, the clones would be a similarly vulnerable population as Dyad literally created them.
It's interesting that the show chose to set some of the clones in Helsinki specifically, because the Declaration of Helsinki is basically THE guidance document on ethics in medical research. Maybe the show was being clever with that.