Unlike your own biological children, the rules surrounding your step-children are very different. Even if you have raise your step-child from birth, you have no automatic rights to that child until and unless you obtain a Court Order that gives them to you. You cannot simply just take the child, if you do you could be charged with kidnapping, deprivation of liberty or similar offences.
If you believe that the child is at risk of harm in the other parent’s care then for the immediate purpose you should contact Department of Communities, Child Safety (DCFS). DCFS can remove the children from the other parent’s care very quickly if the children at at risk of being physically harmed (either through violence or drugs etc). While there is no guarantee that the child would be placed in your care, you can liaise with DCFS and ask for the care of the child or regular access to the child.
In saying all of this it is important to know that you would be considered ‘a significant other person’ in the stepchild’s life and as such you can apply to the Family Court for Orders for access or even custody of the child. In this situation, you would need to prove that the stepchild’s best interests are served by being cared for or maintaining a relationship with you.