Is the one who cuts ties with his brother, who did not give his sisters their rightful inheritance, sinful, especially since one of them is an orphan and has taken on the care of orphans?

Preventing an heir from their rightful share of the inheritance without their consent is an act of injustice and sin. The brother who prevents his sisters from their share of the inheritance must be advised, and the gravity of his actions and the obligation to restore rights must be explained to him. It is permissible to involve respected individuals to mediate with him. If he repents and restores the rights, then all praise be to Allah. Otherwise, there is no harm in shunning him if that deters him, and this is not considered severing ties of kinship.