Is a person rewarded for his kindness to his mother-in-law, and does that compensate for his shortcomings in honoring his own mother?

A wife's kindness to her mother-in-law is a commendable act, consistent with both Islamic law and reason. It garners the husband's رضا (satisfaction) and contributes to the success of the marital relationship. However, kindness to the mother-in-law should not come at the expense of kindness to one's own mother. Parents have great rights, and one must be dutiful and kind to one's mother, even if she is a non-Muslim or calls to disbelief, but without obeying her in shirk (associating partners with Allah). As stated in the Almighty's words: (وَوَصَّيْنَا الْإِنْسَانَ بِوَالِدَيْهِ حُسْنًا وَإِنْ جَاهَدَاكَ لِتُشْرِكَ بِي مَا لَيْسَ لَكَ بِهِ عِلْمٌ فَلَا تُطِعْهُمَا) (Al-Ankabut/8), and His saying: (وَإِنْ جَاهَدَاكَ عَلَى أَنْ تُشْرِكَ بِي مَا لَيْسَ لَكَ بِهِ عِلْمٌ فَلَا تُطِعْهُمَا وَصَاحِبْهُمَا فِي الدُّنْيَا مَعْرُوفًا) (Luqman/15). And from Asma bint Abi Bakr, may Allah be pleased with them both, she sought the Prophet's (peace be upon him) fatwa regarding maintaining ties with her polytheist mother, and he said: "Yes, maintain ties with your mother." Combine kindness to both your mother and your mother-in-law. If the two conflict, do not prioritize your mother-in-law over your mother, provided that you do not spoil your relationship with your husband.