amom,
I don’t think anyone would object to your husband’s and yours dedication to Torah if you are using your own funds or money from someone who dedicated them for that purpose. Kol Hakovod! Your M-I-L may be different as she, understandably, is looking for your benefit and she may be thinking ahead at how your family will live. As Torah is supposed to be sweet for everyone, maybe you can find ways to make your MIL happy with your husband’s learning. Depending on your circumstances, Either invite her as a honored guest when he gives a dvar Torah to public; or start a class for ladies that includes her; or spend 2 out of 12 hours to learn a profession or an internship at some job so that she can see that her grandchildren will have food to eat.
Back to finances, when I said “using your own funds”, there are different views here. My personal view is that this should not include using public funds dedicated for poor people, such as foodstamps, welfare, unemployment, tax credits. Someone who volunteers not to work, for whatever great reason, should not depend on charity, but many people (and here too) disagree.