Lulu, you are awesome!
Good on you for even contemplating it

I can totally understand why you would want to reach out and help D - the story does tug on the heart strings, i do however agree with Cai in that you do need to put your family first and consider how this will effect them
Also Oshani makes a good point about D influencing/enticing your DD to leave school due to finacial freedom .

HOWEVER - if i was in the same situation i would probably give it a go, if D returned to school, could your get FTB for her? I know this wouldnt be heaps, but would help a little...
Could you speak to the family and see if financially they would contribute towards her care? although it sounds like the father is not likely to agree to that, BUT did he work? if not he got the single parent pension i assume? he would loose that i guess, might make him reconsider the living situation.

BG also makes great suggestions about a contract and it is something that definatly should come into it if she were to stay, also how about getting the girls to also set some ground rules re personal space and things, it can all start out with the best intentions, but a tshirt goes missing or deodorant is used, petty i know, but these things can turn into WW3 without good boundaries from the start.

HTH Good luck with your decision