I want to file papers stating that if something happens to my sister I get custody of my niece how do I do this and what papers do I need to file?

You can't. Children are not property.

Where is the father? If something happens to the mother, then the father could claim custody even if your sister leaves instructions.

Your sister can state in her will that she would like you to have guardianship if something happens to her, but that is not binding.

My sisters locked up the Dad Has CPS problems the newborn baby lives with me how can I get custody?

Are you altoalarcon1? If not, please start a new question with more details.