Is a girl sinful if she cuts off her maternal uncle's sons because she refuses to shake hands with them?

The sons of the maternal uncle are not Mahrams, so it is not permissible to shake hands with them. Shaking hands with non-Mahrams is forbidden according to most scholars, and their anger and threats do not make permissible an unlawful act.

Explain to your maternal uncle the ruling of the Sharia regarding the prohibition of shaking hands with non-Mahrams, and that cutting off ties of kinship is among the major prohibitions. Continue to maintain ties with him in a way that does not lead you into the forbidden. Everything that is customarily considered maintaining kinship ties is permissible, such as visiting, fulfilling needs, and corresponding.