Childhood is not idyllic. For many, if not the majority, it is far from being that.
What would help if you could tell us when this happened. These days young children are exposed to sexual influences far younger than ever before. Under such influences your cousin could have fallen victim to influences she could not control or even understand.
How much older was your cousin than you were at the time?
Child abuse takes many forms, in my case it was piano teacher who battered me, always over the chest, every time I made a mistake. He was the church organist and my dad was the minister!
One of my brothers finally stood up to the bastard and then we moved away.
You need to ask yourself if you seriously feel your cousin abused you. If your answer is yes, then you have recourse through the courts, but bear in mind she could have been just another prematurely sexualised kid that really didn't know what she was doing, or why it was so wrong.