Her Anger Here is Misplaced

Question :

I already have a child and my wife is again pregnant. We invited our mother to stay with us, and she came bringing along with her three of my brother's children, which has taken a heavy toll on our budget, and which forced me to borrow money. If I support my mother and send back my nephews, am I undutiful to my mother? She is insisting for me to take care of her grandchildren. And is the following saying correctly attributed to 'Ali: "Whoever angers his parents is undutiful."?

Answer:

If your budget is weak, then you are not obligated to support your brother's children. You can excuse yourself in front of your mother by declaring yourself incapable of supporting them, reminding her that Allâh does not burden a soul with a burden greater than it can bear with what He has given it. When you do so, I don't think that your mother will be angry, because her anger will be misplaced if she remains angry even after you have explained your situation. So don't worry: no sin is upon you for this.

I am not sure that the quotation you mentioned is correctly attributed to 'Ali, may Allah be pleased with him; nevertheless, its meaning is true. Someone is undutiful when he angers his parents without a Shari'ah reason, this is because keeping good relations with them and being dutiful to them is achieved by them being pleased and finding your company pleasant.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 7 Pages 363-364