Stamp duty

My mum and sister are joint on the house title.
My sister,mother and myself all live in this house and my sister is moving out and wants me to pay the outstanding loan of 70k but wants the loan to stay in her name .
I need security so i asked to be on the title.
My mum is on disability pension and i am on single mothers pension.
What would be the stamp duty cost or any cost for me being put on the title?
And will i be excempt from stamp duty if i am on a pension?
The house value is around $600,000
The property is my main residence and my mums.
Will me going on the title effect her pension or mine?
Or what would i have to pay if they both stay on the title and i am added.??