I have read over what you all have said and some of the sticky topics at the beginning. I also spoke with my husband over the lunch hour (friday afternoon here in NZ) and have come up with this.
1 - My sister has expressed interest in moving out to me and to the parents. She always get excuses why she can't when speaking about it to her father. She is waiting until she graduates and finds a job. I will continue to express my support to her, but at the end of the day I do not want to fix the situation for her or control her decisions. I have my own life to lead and the best I can do is support her when she needs it.
2 - The rest of the family. Meh. They know the situation and what they choose to do with that information is up to them. I will concentrate on the positive, non-toxic relationships and keep the rest at arm's length.
3 - I am moving home for a variety of reasons - I am embarking on are a new marriage and in-laws (who are all kinds of awesome) and a new business - both of which need my full attention. I will go to counseling to make sure that any residual sh*t from my mum doesn't spill over into the relationships that I value. I will definitely consider al-anon and won't take the god thing too seriously.
I tend to be over-analytical. I am really digging this forum - cheers for all your help!