the mortgage needs to have the brothers name on it and i believe the mortgage company insists on all names on the deeds if it was purchased 2or 3 ways..
Unless he can prove he had the mortgage in his name (not necessarily paid it) with the fact he has moved out i would of thought he was on dodgy ground.
Only way to resolve this is with a solicitor....A.S.A.P;
Dont take second hand advice or listen to heresay, been there done that...cost me a lot of money, leave it to the professionals.