Is it permissible to boycott the paternal uncle's wife and her daughters and not visit them, if they cause harm and injury, in order to avoid problems? And is this considered a severance of the ties of kinship?

The default assumption for a Muslim is blamelessness and the obligation to think well of them. Therefore, if the accusation against your uncle's wife of practicing sorcery is without proof, it is forbidden. However, if you are certain that she is practicing sorcery, this is a grave evil that does not preclude maintaining ties with her to an extent that does not harm your religion or worldly affairs, along with the obligation to advise her. If she does not heed the advice and shunning her would deter her, then shunning her becomes obligatory. It is also permissible to shun her if it is highly probable that harm will come from her, for scholars have unanimously agreed on the permissibility of shunning for more than three days for anyone whose interaction brings about a deficiency in religion or harm to one's self or worldly affairs.