You say that your sister's hairs became stiff when she shaved them with an electric razor. However, shaving in itself can't make the hairs either coarser or stiffer. It may well be that the cut hair may feel more bristly when it first begins to grow out, but when it gets a bit longer it may appear softer.
There is another possibility, however, and that is that your sister's hair growth is hormonally driven and it's the male hormones which have caused the hairs to get coarser.
Plucking can work very well for some women whose hair growth is NOT hormonally-driven. The hair may well come back a bit finer. However, where the hair growth is hormonally-driven, plucking can have the opposite effect as blood rushes to the site of the plucked hair in an attempt to repair the follicle. The blood carries with it an increased amount of androgens which affect the surrounding follicles also, stimulating male-type hair growth.