If they do not have security over your mothers property they cannot do anything at the moment on that.
Has the FOS found that the CBA was negligent?
You say they paid you $10,000.00 what was that for an admission of guilt for doing the wrong thing?
If it is found that you owe the money you will be liable to pay even if the property is sold for less.
The bank can enforce the mortgage against you personally. Because your mum has passed away the guarantee is at an end.
They are not registered on her hose and they cannot at present do anything.
But if they get judgement against you then they can force the sale of your mothers house as you are an owner of a half share.
Your brothers share is unaffected and the only thing they can take is the equity because the mortgage to westpac has to be paid out as it is secured.
But the equity is only form your share.
Because your mum has passes away the guarantee is no longer in place and her estate cannot be held liable. Also if you are not the owners the estate is finalised.