They normally give 3 months now if it's your mother in law. What you will do is extend her for 2 months as per the advice of Immigration then before her visa expires, you can extend her for another one month seeking approval from the manager. Then another one month, extend her again until you reached 6months (maximum allowable stay for visit visa).

When you get the approval of the manager, make sure you bring a returned confirmed ticket for every extension.