Your poor little neice, the things you say are choking me because it is just echoing everything I have said about my little 7 year old one, and I only have the one to worry about. You must be doing your nut. You just want them to be able to be children don't you? They'll never forget you for it, when they hit their teens and they need support cos their little heads are confused and angry because of the childhood they had they'll know they can depend on you hun, just being a constant in their little lives is enough.
Another idea I had is to contact your local Council (are you in UK - only you say "mom" and not mum and it confused me! LOL). If your sis is in social housing then the housing authority might be able to offer family support.
I live in Oxfordshire and our local Council and housing authorities offer family support, voluntary workers and others types of suport which might be of use to you and your sister. Our family support worker fits in with the anti social behaviour team because often the families that need support are also perpetrators or anti social behaviour, the housing associations have floating support teams who will offer support (help with shopping, sorting out financesm, budgetting, childcare, etc) to those finding it hard to cope, but who may not necessarily be at the stage where social services need to be involved. They will also be able to involve other agencies like youth workers, counsellors, etc if required.
I really feel for your hun, remember that we're on here when you are climbing the walls and need to vent xx