In order to adopt your sisters baby, you will need to file, or have filed by an attorney of your choice, a Petition for Adoption .The Petition , most likely, can be file after the birth of the child. You did not indicate a due date for the child and while it is important to plan ahead, there are some things that can't be done until the child is born .
There are a number of things you will need to ascertain prior to the birth of the child and these include: 1. Whether both your sister and the biological father are in agreement with this adoption; 2. The procedure(s) in Missouri for release of baby to your custody; 3. The procedure(s) that each prison will expect you to follow for obtaining consent from the mother and father.
This type of adoption is called a 'related' adoption, and though these are sometimes viewed as less complicated, your situation is somewhat more complex because of the facts you laid out in your question. Given that you are an Illinois resident and the birth of this child will take place in another state and that the mother and father are both incarcerated, there are a number of issues that need to be addressed, and unless you are well versed in Illinois adoption law, it would be most advantageous to contact an adoption attorney to assist you in this matter.
I would be glad to discuss this further with you, and answer any additional questions you may have.
Sara Howard, Attorney at Law