Is what the sister does, removing her eyebrow hair with tweezers monthly, claiming stress and inability to control it, considered unlawful plucking (nams), knowing its prohibition and that she covers her face?

It is authentically reported that the Prophet (ﷺ) cursed women who practice tattooing and those who have it done, women who pluck eyebrows and those who have it done, and women who artificially separate their teeth to enhance their beauty, thus altering Allah's creation. The Nāmiṣah is she who removes facial hair with tweezers, and Nimāṣ specifically refers to the removal of eyebrow hair to thin and shape them. The reason for this prohibition is the attempt to alter Allah's creation, and it involves deception and fraud. Therefore, reducing eyebrow hair falls under the forbidden Nimāṣ. If the anxiety you claim reaches a degree where she is not aware of what she is doing, then she is in the ruling of an insane person, and there is no accountability upon her. Otherwise, she is sinful. To deal with her, she should be forbidden from this wrongdoing, and the ruling should be explained to her. If she does not desist, then boycotting her is permissible if it is believed that this will bring her back to the right path. If boycotting does not benefit, then there is no need for it; continue inviting her to guidance and supplicating for her.