@AmiB,
What is the thing that most frightens you?
In my state, Massachusetts, the CPS generally does a good job. There are a few sensational exceptions where the CPS messed up. But in the vast majority of cases CPS can connect you with services and will be generally helpful.
My advice (if I can give advice) is to talk to the CPS openly. Because you are 18 years old, your parents don't have control over you.
I would hope that you can find a good counselor first. I think that would make the entire process easier and make you feel less uncertain.
If you talk to CPS you will be assigned a trained social worker who will know what to ask and how to get services. I know several of these social workers... they are good caring people. The
social workers often work with families... rather than taking the children away. What it could mean is that your parents and siblings will have mandated therapy (which could be a good thing).
I understand that doing this is scary. It could also lead to something better for you and your siblings.