Is my participation with my family in going to soothsayers and charlatans to find out who enchanted us—due to the problems that occurred to my brother and his wife after eating sweets from my mother's friend—considered unlawful, knowing that they are aware it is unlawful, but they are angry?

It is not permissible to go to sorcerers, charlatans, and soothsayers. Whoever approaches a soothsayer or a diviner and believes what he says has disbelieved in what was revealed to Muhammad. And whoever approaches a diviner and asks him about something, his prayer will not be accepted for forty nights. It is not permissible to accuse anyone of practicing magic except with clear evidence. You should go to someone who recites the Quran and performs Ruqyah with the legitimate incantations. You can also perform legitimate Ruqyah yourselves, as the Prophet, peace and blessings be upon him, permitted Ruqyah with the Noble Quran. There is no harm in incantations as long as they do not involve polytheism. If what afflicted you is an evil eye from a woman, the cure is for that woman to perform ghusl (ritual bathing). If you did not cooperate in going to these people and advised against it, then there is no sin upon you.