Is refraining from speaking to a maternal uncle who accuses one's mother of theft and causes her harm considered a severing of kinship ties, especially since the mother did not request it?

Your maternal uncle severing ties with his sister is a reprehensible act and a breaking of kinship ties, which is among the major sins that prevent entry into Paradise. His action does not permit you to sever ties with your maternal uncle. The best ways to maintain kinship with him are to advise him gently and wisely, enjoin him to uphold kinship ties, forbid him from breaking them, and strive to reconcile him with his siblings.