What is the ruling of Islamic law regarding shunning a female relative who practices sorcery, given that we have proof of this, and knowing that her husband is unaware of her actions or that he is bewitched, and that maintaining contact with her causes us harm?

If their visit causes you harm, there is no sin in not visiting them. You can maintain ties with the sorceress's husband by phone, sending gifts, inviting him to your home, and similar means, because cutting ties of kinship without a valid reason is impermissible. If you can advise this woman and remind her of Allah Almighty and the danger and sinfulness of sorcery, then do so.