there are many relations in this world. every1 need different respect, trust and behavior. u read the sexual material and got excited with the most nearest female u found.
look, im not saying that u r wrong, b/c its quite natural but brother, remember ur mother leaves u two alone b/c she has trust on u and ur cousin.can u decieve ur mother?
ur cousin may consider u as a brother, as she is living with u people for so long, please don't break her trust by thinking such things about her.
divert ur attension when u have these nasty thoughts. u r namazi mashallah, ask tauba for ur thoughts from the almighty and concentrate on healthy activities of life.
ur mother should also think about it b/c its really a serious issue.she should think of ur cousin's marriage as soon as possible b/c in our society its not acceptable that two adult cousins live for the most of the day alone in one place. also islam strongly condems it.