Hi,
The house can be put up for sale once the legalites of the legal owenership have been completed.
With regards to CGT, your brother will still be liable on the transfer of his share even if the house is not sold as this is not his main home.
He will be liable in the UK as its a UK property but may also be liable in Australia and he should check himself with the Australian tax authorities.
More guidnace is at
Report and pay your Capital Gains Tax
Thank you.