can anyone advise please ... immi website has completely baffled me!!

We have a 175 visa and are still not in Australia yet.
On a drunken night over christmas my hubby confessed to his sister that we would be moving to Australia this year, she was in floods of tears is 44, a special needs teaching assistant who has nothing to do with their mother, their dad is still alive though.
Now, she wants to move with us. Can anyone give me any advice on this?

I've looked at the last remaining relative visa but i don't think she qualifies for this.

Any help would be appreciated, i'm so excited at the thought of her being able to move with us.