How to react when my step-mother did this?

Assalamualaikum. My mother died in cancer when I was six years old. At that time, my father was 35 years old and now he is 46. He has plenty of wealth. Few years ago, he married again, he married a 15 years old girl who is my step-mother actually. She is now 18 years old. As my father is a busy man, most of the time he passed outside the house. So, my young mother and I pass a lot time together. As we are close in age, she prefers me to intimate. He hugs me and gradually she involved me in sex with her which I could not distinguish. I could not differentiate which was the hug as a mother and which was the hug as illegal intimacy. However, now, I have fallen in her love. I want to marry her. But how can I do it Islamically? I know, Islam is a religion of peace and it would bring a good solution. Please, suggest me. I want to get rid of such situation.