The reply of Ayatullah Mahdi Hadavi Tehrani:

Determining your duty in the above mentioned case is dependent upon the condition of your mother, yourself, and your spouse.

1.             If leaving the spouse will result in them falling into sin, then it is not allowed [to leave them].

2.             If abandoning your mother will cause her serious harm, it is not allowed [to abandon her].

3.             If your situation is such that abandoning your mother will cause her serious harm, and leaving your spouse will result in her sinning and you have no choice but to pick one of the two options, then you are free to pick either of them. Although, it is preferable and better to take care of your mother, this may even be your decisive duty. Hence, it is an obligatory precaution that you do what is necessary to stop any serious harm from reaching your mother.