frank i dont know about buying online, but i just got mine from one firstbank branch, no stress ,just paid my 1k and went with my pin.
the internet system in nigeria is not too safe even the internet laws and being reviewed now. i hope ur sister doent hav alot of money in that atm, cause in nigeria nything can happen especially when u punch in delicate details like atm info. on the internet. so be careful ...... go to the nearest firstbank branch around and get the pin.
jah bless.