Speak to a lawyer. If there is no official document giving her brother legal custody of your son, technically she would still have legal custody at this time, but he is acting as the custodial guardian. If your parental rights were never severed, you should still be able to have so say over what is going on, but if she had your parental rights severed while you were in prison, it will be much harder for you. You will most likely need a lawyer though since you don’t know the specifics of what her brother has at this time. You will need to find out who has legal custody and petition to get custody of your son.
Is there any way you can get in touch with her or her brother and tell him that you want you son back? You could try that first.
Just curious, but I thought you were a girl from your other questions and already had your child with you.