How can you stop the father of your son from taking him?

Answer 1

You cannot prevent the father of your son from taking him without a court order. If you have concerns about your sons relationship with his father or your child's safety you need to contact the courts.

Answer 2

Your sister has no parental rights over your son (unless a court has given her these). If this is the case you can simply prevent her from having access to your son.

However, if she has been given rights by a court and from the sound of it she has (and this means your access rights have been removed or limited because of something you have done). The only thing you can do is get an attorney and go back to the court.