Is the inability to eat, despite medical treatment, due to the evil eye? And is it permissible to take water from my maternal uncle and his family's ablution, or is it sufficient to use the water remaining from their juice for bathing?

Exaggerated fear of the evil eye or attributing every matter to it is not appropriate. Its cure is easy with legitimate ruqyah (definition) (incantations) along with reliance on Allah. A Muslim should take precautions to ward off the evil eye with the prescribed adhkar (remembrances).

It is not permissible to accuse anyone of the evil eye without proof, because having ill-suspicion of a Muslim is a sin. Taking something from the belongings of the one suspected of casting the evil eye and placing it in water has no basis in Islamic law or reason. Rather, what is authentically reported is the washing of his limbs and any clothes that touched his body.