The surface story is :
He fancies the younger girl.
The sister is older and is getting it on, so they listen.
He spies on the sister and then ends up in the cupboard spying on her.
He then ends up avoiding the younger girl.
Younger Girl in typical manner goes off with some other chap called Neve and sleeps with him.
He then runs off, and spys more on the sister again.
At some later date, the sister finds him in the cupboard and they end up sleeping together.
He then confesses to her about sleeping with the sister yet wanting to be with her.
She probably hates him, but it's all her fault so...
The surface story is : He fancies the younger girl. The sister is older and is getting it on, so they listen. He spies on the sister and then ends up in the cupboard spying on her. He then ends up avoiding the younger girl. Younger Girl in typical manner goes off with some other chap called Neve and sleeps with him. He then runs off, and spys more on the sister again. At some later date, the sister finds him in the cupboard and they end up sleeping together. He then confesses to her about sleeping with the sister yet wanting to be with her. She probably hates him, but it's all her fault so...