I think you are amazing for considering this & what follows is just my experience & opinion.

My brother is 5 years older then me & so while I wasn't as young as your littlies I was about 8 or 9 or something when my brother went majorly off the rails. My mum being the women she is also left the door open for him, this doesn't mean she would put up with his **** while he was under her roof but he was always welcome IYKWIM? anyway as a result he would bring home friends whose family did kick them out & often we would have at least 1 or 2 of them living with us at once. For me it was a positive experience.

I suppose what I am trying to say is that if you don't think it will have a negative effect your younger children then I would go for it, I would like to think if it was me I would be able to open my home like my mum.

I do agree with BG & the others that mentioned there needs to be some clear boundaries, chores, bedtimes, impact on DD1 etc...

Some practicalities she should be eligible for something from centrelink, if the dad isn't willing to cooperate the mum should (from memory parent had to sign a form confirming they had been kicked out of home etc... but it could have all changed), also if you think she should at least get year 10 maybe talk to her about TAFE, make that a condition of staying.

HTH & GL whatever you decide.