My sister has a 9 year old child. My sister and I live in the same home and have so since my niece was born. I provide her shelter and food. Is there any legal way I can become a legal guardian and my sister still remains a natural guardian?

There is no way both you and your sister can be the legal guardian or legal parent of the child. The only way you could become the legal guardian or legal parent is if both your sister and the father of your niece agreed to give up their legal rights as guardians or parents.

You would then need to go through the adoption process, including obtaining a homestudy and finding an agency or lawyer to provide relinquishment documents for your sister and the father of your niece to sign.