Islamically there is an obligation on all of us to take care of those in need and to assist, help and comfort them to the best of our ability. If your wife is the only child then it becomes a great duty and act of charity to help and treat your mother in law like you would treat your mother.
Legally and culturally she is not your mother but the reward of the good you do is immense. The inlaws of Prophet Muhammad صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ were the ones nearest to him.
Shaykh Yahya Ibrahim answers and explains...
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