My 18 year old step daughter has asked me to adopt her. She lives with her father and I and we reside in the state of South Carolina.

As she is now legally an ADULT any adoption would have little meaning.

Yes you can, although she is no longer a minor and there is really no reason to do it. You could just include her in the will.

But if you want to, and she wants it, and mother wants it, yes you should be able to.