Legal Questions

Is it possible to obtain custody of my sister's minor child while she is away in another country for an extended period of time?

I have a sister who presently resides in Maryland. She came to have her baby in Chicago, and unfortunately the baby had complications. Because of the baby's complications, she has sent an application to immigration for an extension to stay on medical grounds. She now wants to travel to Nigeria to come back later in the year but she cannot travel with the child because of his medical condition. I want to have custody of the child when she is away to oversee his medical care. Can this be done?