it seems fair enough - the house doesn't actually belong to your family, it is there to provide low cost housing for those that require it (i assume)... it was quite nice of them to let the property pass to you from your mother but you can't keep passing it on indefinitly can you
unfortunately it sounds like your brother will have to apply in his own right or try to find somewhere else. The best place to find more info would be to carefully read your tenancy and see what the rules and regs are within it - the association doesn't have to go beyond this in anyway (though i imagine they went beyond the contract when your mother passed away to allow you to stay, seems to be you have had a good turn done for you once but they won't do it again)
Good luck to you both