I think that having estrogenic properties is not the same as having actual estrogen. It appears that moringa acts in some situations to increase estrogen/progesterone and in others to decrease it. Thus it's actions might be estrogen modulatory. In truth, I really do not know enough about Moringa to offer advice one way or another. Apparently, neither does the scientific community and likely neither does her doctor.
If your sister wants to take the "better safe than sorry" route, their are many, many good items to use to fight cancer. For example, see these two posts: