At 6 years of age, I assume she is in school. Talk to her teacher. Her teacher should know of a helpful mom or dad of another student in your sister's class who can help you with these questions and guide you along the way. Or ask one of your friends who has children, or a trusted friend's parents for some guidance.
Whatever circumstance your sister has been through that lead her into your care probably needs to be addressed by a professional to help her to cope. Speak to the social worker in charge of her case or to her school counselor to find out different counseling and therapy options available in your community.
I would also suggest taking a parenting class. Many schools, libraries, and churches offer courses. This will help you to know developmentally appropriate ways of interacting with your sister as well as kind of catch you up on how to be an effective parent. It is wonderful of you to step up and care for your sister, best of luck to the both of you.
State Certified Early Childhood Special Education Teacher
New Mom as of March 2009!