And how old are you? With a "child" who is 30, one can assume that you are at least 50 if not older, so I wouldn't try and play the "age" card with the courts.
I don't mean to sound harsh, but I am thinking of the children here. They've lost their father and it sounds as though they're losing their mother which probably means that they had a pretty rough life for three years after their father died. Then a year ago, they were left with their, I assume, Mother's Aunt and Uncle who are willing and able to take on the burden of raising two young children.
I understand that these are your late brother's children and as such, they mean a great deal to you, but I would ask that you set aside your anger and your grief and think of what is best for those children. You are already struggling financially and you have a full house with two adult children, one of which is handicapped and a grandchild. Can you afford to add two more people to your household? Is your house even large enough?
Instead of fighting for custody...how about instead, petitioning the courts for "family" time...Spring Vacation, every other Christmas, Summer Vacation? To be perfectly honest, given your personal situation, you will have better luck with the courts.
As for the whole "immeadiate family" concept..."blood" isn't everything.