My question involves criminal law for the state of: IL

A few months ago, a 15 year old was reported missing/runaway and from what her grandfather has told me, this will be her 5th time running away. Each time she has been linked to my 27 year old brother. He is married with 2 kids, but because of marital issues he is staying in my parents basement. Since she went missing, he has displayed many strange behaviors such as changing the locks in the basement, purchased a camp stove, and brought in a 55inch television which I later found out through one of his former friends that it is hollow and does not work, it's what he uses to hide the girl when my parents go to the basement. There have been random things of mine that go missing, and I have even found her bra in the bathroom. A few days ago, while my parents were out, my boyfriend found her hiding in our basement. As he went upstairs to notify someone she left through the window. She left her bag with items clearly showing she had been staying here for some time. The police were notified and brought my brother in for questioning but he was released that night. He denies everything and claims we are all lying against him. I fear for mine, my 2 year old daughter, 10 year old & 15 year old sister's safety as I know he is a sexual predator. He has hit me several times in the past. When I was 11 he used to make sexual comments toward me and would tell me all the time that he wishes I were "his girl". My parents know all of this information and have not taken any action. I feel that they are in denial and can not comprehend that there is something wrong with this "man". I don't know what else I can do. Please help, I need as much legal advice as I can get...