How do I get my mother in law to let us help manage her finances?

MIL has run out of money and about a year ago asked her two kids for help. (She does get a good SS payment each month and some public assistance). At first this was split evenly and the amount was manageable until my spouse's sibling stopped contributing. Now it can be very difficult for us some months as my spouse works in sales and commission is very unpredictable. MIL asked me to help her apply for renewal of her aid, and through that process I became aware of her monthly expenses.Basically I found that it would be reasonable for her to manage all her expenses with us contributing half of what we have been. Would she have to be a little more careful with some things? Yes but I do not feel the limitations are unreasonable. I explained our financial needs and at first she was willing to work with me; we basically did the checkbook and paid bills together. It was very beneficial for me to know the cash flow so we could still help her but also pay our own bills when commissions aren't good. After about a month of this, she has told me she doesn't want my involvement anymore, it feels too confining to her. I responded calmly, again explaining our need to manage both her needs as well as ours with a fluctuating income. But inside I am very frustrated and yes, a little resentful. Given both our circumstances, I feel its unfair of her to expect a check from us monthly without us knowing what is really going on. How would you deal with this situation? I am thinking of writing down a budget that would show her how she can pay all her monthly expenses, including rx, groceries, and some spending cash, and need only half the amount we've been giving her. We'll be happy to give her more when commissions are good. On the other hand I do not feel like just blindly writing the check every month any more. She is mentally capable, but does find some things overwhelming (like the aid forms I helped her with) and I think is more stressed with finances in general. But she does not want to give up control. And at this point, that is not my goal. I simply need to know what the cash flow is, so to gauge exactly how much assistance she does require at the end of the month. What should my next steps be?