Do we have to have the person who has legal custody sign a paper for guardianship being changed from one guardian to the next?

Dear South Carolina, No, your sister cannot give you joint guardianship.  Custody and Guardianship are legally determined based on petitions filed in court and by the determination made in court based on the best interests of the individual over whom custody and guardianship is sought.  With that said, your brother's grandparents could not "give" your sister guardianship.  They could have petitioned the court for her to have guardianship (or she petitioned for herself), but it was the court that awarded guardianship.  If there is no court  legally appointed guardianship in place, your sister is merely sitting for your brother. As guardian (an official court appointed guardian) your sis can appoint, without court approval, a short term guardian....someone to act on her behalf when she is unable or unavailable to...check your state's rules for specifics as laws can vary from state to state and I practice in Illinois.  Your sister can also designate a standby guardian in a signed and witnessed writing....someone to become the Guardian upon her death.  Here again please consult an attorney in your state to learn of the specifics of how this works in South Carolina. The fact that your brother lives in your household does not entitle you to be a joint guardian but it does provide you with a wonderful opportunity to be a great blessing and help in both his life and your sister's....so great that it will make your brother's grandparents ashame that they were ever mad.  So go on and prove them wrong-serve your butt off and enrich the lives of those in your household. Your own life will be enriched as well.  Best wishes to you.