Custody of my sisters child

Me and my sister have no relationship because she's an awful person and we will never have a relationship. I have custody of her baby for the baby, not her. She raped a dude, got pregnant and is served a 10 month sentence in prison. The fact that her time was so short is terrible and shows how the justice system cares about rapist. So, father signed away his rights, sister has no rights and I have the baby and I have go to go back to court Monday for permanent custody. I personally don't know what to let the baby know mess. Mom or aunt. My sister won't be involved.

She's 8 months... I guess I'm scared to be mom because what would I tell her about her bio parents.. And I'm scared for the day we have a fight and she says you're not my real mom...