Guardianship of sibling?

Hey everyone.

I’m 21. I live at home in Chicago with my 15 year old sister. We have another sister, 26, who left our parents’ home when she was 18. Long story short, our parents were very physically abusive when we were younger. My older sister took the brunt of it. Child Protective Services were called once and were incredibly useless (the social worker? interviewed my younger sister and I…directly in front of our parents).

After my older sister left, the physical abuse stopped, but the emotional abuse kept up. My little sister admitted to being depressed and entertaining suicidal thoughts.

I thought about asking my older sister to attempt to petition for guardianship of her, but I’m worried about what the chances of that are. There’s the previous CPS visit, but beyond that government forces were never involved (for the very obvious reasons that they failed terrifically that one incident).

Thank you for any help you can provide!