A child being abused can be any age from newborn and up until you become an adult not living in someone else's home.
Report the matter to the appropriate authorities.
Tell someone, who is in a position of authority and trust. Someone who is not related to the abuser is more likely to help you.
The police.
they may be very emotionally unstable, asking alot of abnormal questions if the abuse was sexually based or showing violent behaviour.
Talking to a school counseler would be a good first step. If you think its severe enough call defacs or the police