When I was a kid, my Dad was a brain tumor patient (frontal glioma) and we DID need to adjust our spending pattern. When one parent is sick, it is reasonable to experience SOME adjustment in lifestyle, and it is NOT necessarilly a bad thing for a child to learn to think of another's needs before their own. None of us suffered for it - life DOES sometimes need to change when one's parent has a major illness. Amazing - when a kids parents are married to each other they live with and are affected by a parents illness and the financial impacts, but we expect them to be totally INSULATED from it if the parents are divorced? It's STILL their parent.
Now, poster, are you on SS disablity? Do you have either language or motor skill impairments? If you are or will be recieving SS disabaility, your children are entitled to benefits under your SS. Be certain to have them apply. And see about having income imputed to her. ALL adults need to have transportation and put a roof over their heads, even if they don't have kids. Most of my friends have homes even after their kids have moved out, having kids in the home is not the only reason to have a house. Most I know believe that they need a house and car whether or not they have had kids, or their kids are now off on their own.
Also, apply for a modification if your surgery or tumor has affected your cognitive or motor abilities in a way that impacts your earning capacity. People just don't get that the mere process of cutting into the brain to remove the tumor (and radiation/chemo, etc.) can create permanant damage to some brain functions, depending on tumor location.