My mother and step-father are giving me my little sister. How do I go about getting custody so I can put her in school?

The simplest way would probably be to get guardianship.  Your sister's father, if he is alive, is also required to receive notice of the guardianship proceeding.  If your sister is age 14 or over, she also has the right to nominate her guardian.  If everyone is in agreement with the arrangement an agreed order can be presented to the Court for approval, making the process more simple.