Is it permissible for me to cut ties with my paternal uncle who commits grave sins and harms me and my family, given that scholars have ruled him to be a disbeliever?

If these descriptions apply to someone, then evils have converged in them. Therefore, you bear no sin if you cut ties with them, or respond in kind, or complain about them to the authorities, for Allah Almighty says: "Allah does not like the public mention of evil except by one who has been wronged," and His saying: "And whoever avenges himself after he has been wronged – those have no cause for blame."

However, this is not advised except after exhausting all means by which you can reclaim your right and deter them from evil. If the transgressor persists in their sin, then advising them and clarifying the truth to them is permissible.

It is also guided to turn to Allah in supplication, for the prayer of the oppressed is answered. The Prophet, peace and blessings be upon him, said: "Beware of the supplication of the oppressed, for there is no barrier between it and Allah."