If my sister gets full custody, will I have to pay child support?

Typically in the case of an adoption, the biological parent will no longer have the obligation to the child. In child custody proceedings the rules are different. Unless the circumstances of the child manifestly dictate otherwise and the court specifically so provides, the obligation of a parent to make child support payments shall terminate when the child: (1) Dies; (2) Marries; (3) Enters active duty in the military; (4) Becomes self-supporting, provided that the custodial parent has relinquished the child from parental control by express or implied consent; (5) Reaches age eighteen, unless the provisions of subsection 4 or 5 of this section apply; or (6) Reaches age twenty-one, unless the provisions of the child support order specifically extend the parental support order past the child's twenty-first birthday for reasons provided by subsection 4 of this section. 4. If the child is physically or mentally incapacitated from supporting himself and insolvent and unmarried, the court may extend the parental support obligation past the child's eighteenth birthday. 5. If when a child reaches age eighteen, the child is enrolled in and attending a secondary school program of instruction, the parental support obligation shall continue, if the child continues to attend and progresses toward completion of said program, until the child completes such program or reaches age twenty-one, whichever first occurs. This can be a complex issue and can vary depending on specific circumstances (bio parent, guardianship, adoption, termination of rights).