She has two siblings - have you talked to them about how their behavior affects her? They need to take some responsibility for their actions, especially when their actions negatively affect other people.
At nine, she needs the influence of both parents. Mom needs to be a role model and stand firm, being assertive when needed. Dad needs to coach and generally affirm her abilities. (Be careful here - you don't want to say "That was a great paper" if she might have agonized over it for hours, but say "You're a great writer" to recognize her ability to do it well.)
Is she the middle or oldest child? Is she naturally reserved and introverted? Does she feel alienated or is she okay with not having close friends?
If she won't talk even if something is bothering her, and she's only 9 (not 19), she's learned that it's not worth it to speak up. Then it's a downward cycle - she isn't heard, so she feels like she doesn't matter, which leads her to feel that she isn't any good. She needs some coaching on ways to express herself.
This is a family issue - she needs unconditional love and acceptance, and possibly professional help. You might want to talk to a child psychologist. At the very least, a specialist will be able to determine how serious this might be with just a phone call. Good luck!