You and your mum stay in Hkg, right?
Your grandmum stay in Aussie, right?
----> how many siblings does your grandmum have?
bcs, it is very important ! if more than one, no hope !!
----> the Age and Skill of your mum and you are also vital.
Over 45 and or with out equivalent to F5 or special skill......least chance !!
And....this excuse not very solid for migration.
Further information, check it via Australian Embassy, located in Harbour Centre, Wanchai (opposite of Wanchai pier).
Get the migration approved first, too early and too soon to talk about uni. (IELTS at least 7.0 or above !!)