Is there a legal paper to sign to relinquish my role as trustee of my parents' trust, leaving my half to my sister?

You do not need a form document to state you are refusing to accept the appointment; the distribution of the funds still goes as the Trust states unless you formally waive that. Be sure first that she is willing to accept the position.

You cannot be forced to serve as Trustee. But once you step away you no longer administer the trust by its terms.