How do I tell my Mom and Dad that I wanna live with my Dad?

My name is Kylie. My mom and Dad were never married but, they stayed together because they had kids. I am 13 years old which means I am the oldest and my little sister is 8. Basically since they were never married they never got divorced they just separated. I still see my dad and so does my sister but, I miss him so much. I have nightmares and I can't sleep. My grades are slipping as a result. Right now he lives at his Mom's house. It's good because my Grandma lives right up the street and he is in the process of getting his own apartment. I don't know how to tell him that I wanna come with him. I also don't know how to tell my Mom. She will probably be angry because when they asked who I would rather live with I said my Mom because I was scared. I don't know what to do with my sister I can't just leave her. My Mom isn't the best Mom and my sister says if I leave she will be sad. We snuggle and when my Mom yells I console her. I don't know what to do I really miss my Dad!