My dad never told me who.my mom was

My dad was a single dad at.14. He never talked about my mom and refused to tell me. All I know about hi mom is she's serving a life sentence in prison but he refuses to tell me a name. Every time I asked he said I don't need to know and some things are best left alone. I have never seen my birth certificate and I have no idea about my family. His dad is dead so he can't tell me anything. We had a fight today. I will say my dad has done everything for me and I love him for it. But when I see girls playing with their mom I wanted to try to find mine. We fought about it and I told him I hate him and my mom would have been a much better parent... He looked hurt and said okay you think that. He went to his room and dragged put this cheap fire proof safe he has. He opened it and gave me three papers. One was my birth certificate, and a women's death certificate. I asked was this my mother and she said yes. They had the same last name which confused me since my father was never married. I asked him about it and he said she's his half sister.. He went to live with his dad when he was 10 and she lived there. I looked at the years they were born... They're 11 years apart. I pointed that out and dad said she sexually abused him from 11-13. She did 2 years in prison and when she got out she was murdered 2 weeks.later. I asked by who and he said "My mother". I said " That's means I'm an incest baby..." He said "Yeah. I didn't want to relive my trauma and wanted to protect you from carrying that burden. But there! Now you know about your AMAZING MOM! I'm going to bed". I know my dad is upset at me for pushing. I want to talk to him... But I have no idea the right words...