Putting someone out

First how old are they, are they over 18,

You said step child, so do both the step parent and the parent agree about "thowing him out"

Assuming you both agree to get them out, and assuming they are over 18, you give then notice, and if they don't move, you file in court for a formal eviction.

As FrChuck said - over 18, here's the law: South Carolina Eviction