How can one ascertain whether a sister's husband has been bewitched or is under a spell, and what is the correct way to deal with this situation?

The fundamental principle is that a wife has the right to separate accommodation from her husband's family, and she may refuse intimacy if he does not provide it. However, if accommodation is provided, she is not permitted to refuse intimacy with her husband, as the impediment has been removed. Otherwise, she would deserve to be cursed and described as disobedient (nashiz).

As for hanging amulets: If they consist of texts from the two Revelations (Quran and Sunnah), the correct view is that it is impermissible to hang them due to the general prohibition, to block the means (to shirk), and the possibility of their desecration. If they are from sources other than the two Revelations, then their impermissibility is unequivocal.

Regarding doubt about magic: The situation can be presented to scholars and religious people specialized in lawful ruqyah, along with frequent remembrance of Allah. Resorting to magicians and charlatans is forbidden.