Must I stop visiting my cousin after discovering his visits to charlatans, especially after his marriage proposal to my sister was rejected?

Going to charlatans and believing them is among the gravest of forbidden sins. And whoever goes to a soothsayer and asks him about something, his prayer will not be accepted for forty nights.

Advise your cousin to stop going to them, for he is committing something forbidden, and you may boycott him if that will stop him.

As for your aunt and the rest of your relatives, do not boycott them. Continue visiting them and maintaining kinship ties, even if they cut you off or treat you badly.