Wow, there is a lot of stuff here, so it really seems to me like you're overwhelmed with issues here. I think your main concern is that you never truly grieved for your sister. I know what it's like to lose a sibling, and I think it's important that you grieve properly. Did you ever have access to some form of bereavement counseling? I think that could really help you at least get some form of closure in this. I know it's tricky and I know you're struggling with everything, but I think you're just becoming so overwhelmed with grief that everything else is made just that little bit harder. I'm so sorry you had to go something has horrific as sexual abuse, but I really don't want you to blame yourself for what happened. The person who abused you is disgusting, and you can't blame yourself for their horrible actions. However, masturbation, whether it came about as a result of this attack or not, is not immoral. Every human being has a sex drive, whether we like to admit it or not, and masturbation is one way for us to fulfill our sexual needs on our own.
Your guidance counsellor was wrong for not getting the authorities involved. Your mum abuses you. Hitting ANY child is totally wrong, and you have the right to live your life free from that kind of behaviour. I think the other problems in your life will sort themselves out once you've dealt with your grief and have set yourself apart from your mothers abusive behaviour.
Good luck (: