What is the ruling on a wife preventing her husband's nephews from seeing their paternal aunt out of jealousy, and the husband obeying her in this, and what is the punishment for this act? And what is the brother's duty towards his only sister, who has cut ties with him?

Your brother and his wife are not permitted to prevent you from seeing and connecting with their children, especially if there is no legitimate Shar’i excuse for doing so, such as fear of the children being corrupted. This matter should be addressed through sincere advice (munasaha), by clarifying the Shar’i ruling to them, and by interceding through someone who holds influence with them. You should continue to maintain ties with the children and treat them with kindness, in compliance with the words of Allah Almighty: "Repel evil with that which is best, and lo! he between whom and yourself was enmity will become as though he were a bosom friend."