If you moved out and rented the property within 5 years of the original purchase as owner occupiers then you are liable for a clawback of stamp duty now.
Your brother should not be claiming owner occupier mortgage interest tax relief if he is no longer an owner occupier.
If the property is sold within 12 months of vacating it as your PPR then not CGT applies. Otherwise some portion of any eventual resale gain is assessable for CGT. For example if you lived in the property for 6 years then moved out and rented it for 3 years before selling it then ((3-1)/6) = 2/6ths or 33% of half (since your brother and you are presumably 50:50 co-owners) of any resale gain will be assessable for CGT using the normal calculations.