such a complicated mess..but first I agree do not throw good money after bad on this matter...your daughter has been through much but this is her problem to deal with not yours...you can over practical help where possible, but the mechanics and financial costs are hers to deal with.. you too have been through a lot and you deserve your own happiness and security too...
She needs to speak to a child psychologist regarding the boy and his behaviour and agree some kind of treatment, clearly his spell in hospital was not helpful, but if he is endangering his sister as well as his mother he obviously needs some quite radical attention.. do NOT feel guilty OR responsible...you can give advice and steer her in the right direction, but DO NOT take this mantle on board, words are easy but you simply must heed the advice given here by many....