my aunt has had custody of my brother for 8 years. Now she wants to sign custody over to my husband and me. Does she acctually have to sign over custody or can she sign a notarized document giving me permission to take care of him???

She can do both. She can execute a power of attorney to allow you to care for him. However, my suggestion is to also go to court and make it legal. File a joint petition to transfer legal guardianship to you and your husband for your brother. The court may not even hold a hearing since all parties are in agreement, but there may be a brief hearing involved, but I still think this is best for the longrun to ensure it is all legal.