let me try to explain in a more "layman" way here.

Why can't use giro?
giro is for local banks (transfer money for all banks within M'sia) only.
since yr sis is overseas, there is no way yr parents can use GIRO to transfer money to her.

TT rate.
currency rate change/float everyday.
for example:
today RM1= 0.18 British Pounds
tomorrow RM1 might be 0.19 Bristish Pounds

That's why yr parents need to check the currency rate each time they need to do transfer.

TT transfer.
u need to do this in bank.
say for exchange rate RM 1=0.1869 GBP

say yr sis needs GBP100, then yr parents need to pay abt 100/0.1869=RM535. (plus some service charge)

hope that's clear.