yeh, by all means though, ring a couple of banks and get their rates on the same day (euro/sterling rates are changing fairly quick now, against your favour). Then ring the brokers and compare their rates.
Then compare all those to the mid-market rate, from xe.com or currency solutions which update more often than google.
Some of the brokers require credit checks, if your sister is on UK voting register or UK resident for >3 years she should have no problem. Otherwise she may have to send in a photocopy of her passport / driving license, certified by a solicitor / notary. In that case currency.ie might be better.
btw just about none of them list their actual rates online, presumably because their rates are different for different amounts, so unfortunately you have to ring them individually.
Ok fine i'll give u the numbers!
currency today 0044 207 183 2291
currency solutions 0044 207 740 0000
sterling exchange 0044 0207 329 9977
currency.ie 00353 1 635 3700