Wow, I am absolutely amazed at all you have been through. Somehow, you have got through all of that and yet you think you have nothing to offer the world. Just the many injured people out there who you would be able to understand! Understanding and empathy is something the world desperately needs and only people who have really suffered can offer it. At this point, of course, you need to take care of yourself ONLY.
You have been badly abused by family and yet you are afraid of telling them how you feel in case you worry them? It is about time they worry. You really do deserve support. I think, given what you have been through you have been very brave - and that you should appreciate all that is good about yourself. (Not even 10 years old, a defenceless child and yet you were defending your sister!)
Here's sending you a hug

and lots of love to go with it -
p.s. sorry, my experience is very limited so if I have said anything stupid, please just put it down to that.
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