I agree with your dd actually, in that my dd has been quite aggressive and it occurred to me recently after reading threads on here about self-harm, that actually I'd rather dd shouted or was rude to me than self-harmed. I haven't self-harmed personally, but have suffered from depression, and I know that turning those negative feelings inwards - whilst more socially acceptable - isn't a solution either. I think your dd is just trying to explain to you that to her, the rage is her way of expressing the same emotions your ds does through self-harming ie very strong emotions, that are difficult for her to handle. But just as you wouldn't 'punish' your self-harming child, I don't think you should punish one who's clearly stated this is her way of expressing the same feelings.
Your dd clearly is unhappy but the negative emotions are coming out as rage. But again, looking on the bright side, it's only her own room and possessions she's destroying, not yours - so quite considerate in a funny way. As it's her room, personally, I'd just ignore the mess - she can clean it up if it bothers her. And don't give her any breakable presents!
As much as you can try to get her to have enough sleep, eat properly (B vitamins are good for stress), and regularly, and take exercise. And try to get outside on sunny days. Little things, but that are all essential to build up her ability to calm herself and stay calm.
And then look at counselling, as others have suggested. And don't forget yourself - it sounds like you must be under huge pressure, with two children acting out in different ways. Look after yourself, treat yourself get counselling for yourself if you feel it would be useful.