What is the ruling of Islamic law regarding how to deal with the case of a sister's daughter who performs strange acts such as spilling sand, garlic, and a brown substance, with the wife suspecting that the sister practices sorcery, especially after conflicting pregnancy test results?

It is not permissible to accuse a sister of sorcery without proof. Rather, one must fortify oneself with ruqyah ash-shar'iyyah (Islamic incantations) and the prescribed invocations. Her ill-treatment should be met with kindness, for Allah Almighty says: "Repel evil with that which is best, and behold, he with whom there was enmity will become as though he was a devoted friend," and for the Prophet, peace and blessings be upon him, who said: "Maintain ties with whoever severs them, give to whoever deprives you, and forgive whoever wrongs you."