You can work hacksaw blades with a file and with a nail file but things become a lot more involved.
If working with files the blades will have to be annealed by heating to a bright red then letting cool in air, this will make the blades soft.
You then file away.
You then heat the blade up to red again and then dunk it into water.
Clean the blade to a shiney surface with emry cloth then heat gently until a straw colour then dunk it in water again.
Finish the pick off by emrying it down well.
Another way is to wrap somthing like a file or block of wood etc in emry cloth and gently emry away the form of the pick but its a long task and youll use a lot of emry cloth.
A nail file can be used but will not last at all and maybe even only last a few strokes, certainly on a unsoftened blade you will destroy it in one stroke, give it back to your sister and go to the dollar store or pond shop, you willget a set of 5 cheap files that will be better.
That said i reckon the nail file itself may make a pick if made righ
