You've not specified whether the power of attorney is 'ordinary' or 'lasting'.
An ordinary power of attorney gives someone else the authority to act on your behalf but only while you're still able to oversee their actions.
A lasting power of attorney (which is probably what you're referring to) only kicks in when either:
(a) the person who consented to the power of attorney is no longer able to manage their own affairs ; or
(b) that person simply decides that they no longer wish to manage their own affairs (even though they're still able to do so).
So your brother is acting legally if either:
(i) your mother signed an ordinary power of attorney ; or
(ii) she signed a lasting power of attorney and has also attested that she no longer wishes to manage her own affairs.
However if your mother signed a lasting power of attorney but has not attested that she no longer wishes to handle her own affairs, your brother is not empowered to act on her behalf.
http://www.ageuk.org.uk/money-matters/legal-issues/powers-of-attorney/power-of-attorney/