Grandparents and Third Parties: How to Get Custody of a Niece and Nephew Based on Parents' Drug Addiction

My question involves a child custody case from the State of: Illinois and Missouri.

I currently care for my 10 year old niece and 7 year old nephew who I have had temporary custody of on and off since they were 6 and 3. My sister and the kids Dad have both been using drugs for quite some time. Initially I went through the states attorney to get a protective order and temporary custody of the kids but that order expired when my sister was doing better and able to care for the kids. Things have gradually gotten worse, she has overdosed multiple times. I called DCFS in Missouri (where she lives) and they investigated.

DCFS tried helping her but eventually told her that she needed to let the kids go to a family member or they would be removed from their custody and placed in foster care. I took the kids at that time but DCFS did not and would not give me any documentation. She was doing well (not using) for almost a year but recently overdosed while the kids were there visiting with their grandfather. They saw the entire thing.

She is ok but I'm afraid she will try to come get the kids and I won't be able to do anything about it since I do not have custody and I have been having trouble taking them to doctor appointments, registering for school, and I am not able to put them under my benefits. How would I go about getting custody of them? I have a little bit in savings but only a few thousand dollars so I'm not sure that I can afford a lawyer. Any advice?