Dear WobblyWombat,
I am so sorry you had such a horrible person at your home who destroyed your childhood and the one person who should have protected you just looked the other way. Sorry, destroyed your childhood is putting it too mildly

.
I think you and your sister should try therapy together. You both underwent something traumatic and by dealing with it together, you may be able to support each other. I think you are allowed to be mad at your mom.
I know how you feel when you say you have no friends. Most people find me funny. Most people use me as an ego-boost. I am a people pleaser and most people keep me around because they know I'd be useful. So, I really have no friends

.....but I AM getting better at it....with healing my pain. You'd find a lot of friends here. People who really understand your pain....because we sort of have been there.
I do hope you take up writing. It's very therapeutic. Heal from your pain and you'd make friends and maybe find a person who loves you the way you are and love you unconditionally, doesn't matter at what age.
Right now, healing your wounds is the most important part. I hope you can take up something powerful like kickboxing or karate or something like that, because it makes you feel very powerful. You were vulnerable back then. But as dysfunctional adults, we carry that vulnerability. It would definitely make you feel very powerful, make you feel that you can protect yourself (that inner child in you will feel safe).
I wish you all my best to heal from this pain.