Is a person considered sinful if he stops visiting his relatives and asking about them for fear of their harm, and is it permissible for him to pray for their destruction?

What your uncle and his family are doing — their oppression, manipulation, envy, and hatred — is evil, but it does not justify cutting off ties of kinship, for it is one of the major sins. If you maintain ties with them and treat them well, Allah will be your helper against them, as stated in the noble Hadith that whoever maintains ties with his relatives and treats them well, while they cut him off and treat him badly, it is as if he is feeding them hot ashes, and he will continue to have a supporter from Allah. It is best not to supplicate against them, but rather to be patient and seek reward from Allah, as Allah Almighty says: "And that you pardon is nearer to righteousness," and "And whoever is patient and forgives, indeed that is of the matters [requiring] determination." However, it is permissible for the oppressed person to supplicate against his oppressor to the extent of the injustice, such as saying: "O Allah, protect me from the evil of so-and-so however You wish."