She is jealous because her husband cares more about his sister than her

Praise be to Allah.

Firstly: 

The relationship between a man and his family should not get in the way of his relationship with his wife and children. The smart wife is the one who is keen for her husband to develop a strong relationship with his own family and for there to be friendship and love between them. She should not be an obstacle in the way of the happiness of her husband’s family. 

The smart wife is wise in her dealings if she sees some shortcomings in the way he treats his children and wife, and she is wise if she sees him exaggerating in his concern for his family at the expense of his wife and children. So she makes him aware of his mistake by hinting, not by stating it bluntly, and she advises him without hurting his feelings, and shows him the right way without making him feel that he is wrong, letting him feel that she loves his family and cares for them. 

A woman should not think that her husband’s care for his family will make him fall short in his duties towards her and his children. There is a difference between a husband falling short in his duties towards his wife and children and his loving his family. What makes a man care for his family is his love for them, not his negligence towards his wife and children. 

Secondly: 

Your husband has rights and you have duties. Each of you should do that which is enjoined upon him or her by sharee’ah. Sharee’ah cannot make a man stop loving his family because he has got married, and we cannot ask him to suppress his feelings for the sake of his wife and children. They were his family before he got married and they will remain his family after he gets married. 

You have to try hard to deal with your own feelings, and advise your husband when he falls short in his duties towards you and his children. You do not have the right to denounce him for his love for his sister or his family. There is the fear in such situations that the man will say, “I can find a thousand wives but I cannot find another brother or sister.” So beware of making the matter reach that state. 

At the same time we advise the husband to be pay attention to his wife’s feelings and to take care of his wife and children. We also advise the sister to give her brother similar advice. 

Islam enjoins doing that which is in the interests of all concerned, so that they may share in building the family, not destroying it. All of us must do the duties that Allaah has enjoined upon us, without exaggeration or neglect. 

And Allaah is the Guide to the straight path.