Is it possible that my thinking about my maternal uncle and his wealth, followed by his illness 12 hours later, was the cause of him being afflicted by the evil eye, and what is incumbent upon me to do if that is the case?

The death of the uncle cannot be definitively attributed to an evil eye, because the matter of the evil eye is subtle, and the default is the innocence of individuals until there is clear evidence to establish rights. It is most likely that what one's self suggests is a type of whisper (waswasa) and mere speculation, especially since the uncle had been suffering from heart disease for years.