Getting custody of a sibling can be a particularly tricky type of custody battle. Siblings are often close in age, and share one or both parents, making the fight to obtain custody emotionally draining. While a sibling may have selfless reasons for attempting to gain custody -- like if the child's parent is abusive or neglecting, or if the biological parents have died -- the court will want him to demonstrate that he is financially and otherwise stable and independent, and able to adequately support the sibling.
Ask your sibling's parents or legal guardians to relinquish legal custody of the child. If they are aware that they are unable to provide adequate care for the child, they may actually appreciate the opportunity to provide an alternative solution to the problem. If they agree to name you the child's legal custodian, it will make the entire situation much smoother and more pleasant for you, them and your sibling.
File an application for custody in the court where your sibling and his legal guardian live. If the child lives somewhere besides with his legal guardian, you may also be able to file in the court that has jurisdiction where the child lives. For example, if your sibling has been living with you, even though you don't have legal custody, you may be able to file for custody in your local court, depending on your state law. The court clerk at your local courthouse will be able to help you answer questions of jurisdiction.
Demonstrate in court that your sibling's legal guardians are abusive or neglectful or otherwise harmful to the child. A court will usually not take away guardianship from the parent unless the child is actually in danger of harm. In addition, you will need to thoroughly demonstrate your own capability for supporting and caring for your sibling.
To be eligible for a custody case, your sibling must be under age 18 or otherwise legally dependent, and you must be over age 18 or legally emancipated. While unforeseen tragedies can occur that leave survivors scrambling for answers, being prepared is always the best solution. If you know you would want custody of your siblings should his parents pass away, ask them to name you his legal guardian in their will. This will minimise legal wranglings should the unthinkable occur. If you are unable to obtain custody of the sibling, consider filing for visitation. This will enable you to continue seeing your sibling, maintain a relationship and allow you to continue being a positive influence.