I don't think you do. If it's in her power to bring them back, she's ethically obliged to do so.
So if I could bring a dead person by sacrificing say an 8 year old child I would be ethically obliged to do so because it would be within my power to be able to?
Not everyone here agrees that Tuvok and Neelix would be considered dead just like not everyone agrees that they aren't. No matter how you slice it, someone in that episode was going away permanently. Others have already discussed the numbers game. Perhaps Tuvok's family would be more... forgiving of the choice to leave Tuvix as is, but were it my spouse who was left like that when the technology exisited to return two people to their orignal lives, I'd be pissed.
It affects more people than just Tuvix. It would have been interesting to hear Tuvok himself weigh in on it.