Is there a sin upon one who cuts off visiting relatives to avoid problems, especially if mixing with them leads to witchcraft or gossip?

There is no harm in preventing your sisters from communicating with your paternal uncle's daughters if you fear corruption for them. Similarly, there is no harm in discontinuing your visits to your paternal uncle's house if such visits cause problems, and sufficing with phone calls or messages. This is not considered a severance of kinship ties. One must be cautious about accusing anyone of sorcery without evidence, and what the jinn mention should not be believed, for it is the nature of devils to lie and ignite discord among people.