As a mother of a teenager who suffers with Serious depression, anxiety and some OCD, I beg you to tell your parents or an adult you trust. You are young and need support to determine the help you desperately need. Don't be ashamed about your sister. Like you said, she was young and so were you. You may feel guilt but don't let it control your life. Let it go and work towards getting better and please talk to your parent(s).