What is incumbent upon a wife who accuses her co-wife of witchcraft without proof? And what should the husband do in this situation?

The default assumption for Muslims is safety (innocence). Therefore, it is not permissible for the second wife to accuse the first wife of witchcraft without proof; otherwise, she is sinful. The husband should advise her with wisdom and explain to her the danger of ill-suspicion and bad manners, and that such actions could be a cause for entering Hellfire. If there is a strong suspicion of witchcraft, then they should resort to Ruqyah (Islamic exorcism/healing). If the sister-in-law was the one who relayed the information, she has committed backbiting (namima), and she must be reminded to repent and seek forgiveness from the first wife, provided that doing so does not lead to a greater harm.