I don't think other banks will want to know, unless your brother has a well paid job. Would it not be much easer to change to a lower rate with your current bank?
As the houses LTV (loan to value) increases you can get a better rate.
eg for AIB:
ECB plus 0.6% for LTV under 50%
ECB plus 0.76% for LTV between 50 and 80%
You have to go looking for these rates, the banks will not give them off their own back. Just write to the bank stating my house is now worth X, I want rate Y. If the value of the house is obvious they will just give it to you, otherwise they might want a few printouts from My Home of similar houses in the area or at worse valuators report to prove it.