Is it permissible to visit relatives known to practice sorcery, in order to mend ties and avoid cutting off family relations, despite fearing harm to oneself and one's child from them?

It is not permissible to accuse a Muslim of profligacy without proof. If you are certain that the woman causes harm through magic and you fear her evil, there is no sin in avoiding her to protect yourselves from her wickedness, and this is not considered blameworthy abandonment. The husband and his children should maintain ties with their brother and uncle via phone or similar means without interacting with the woman, because the brother and uncle are relatives with whom ties must be maintained. Maintaining kinship ties has degrees, and the lowest of them is to avoid abandoning them and to connect with them through speech. The questioner must observe hijab in front of her husband's brother because he is a non-mahram to her. The wife of the husband's brother and their children are not among the questioner's relatives with whom she is obligated to maintain ties, but they have the rights of Muslims and their relation to her husband.