system
Given the following post, respond as if you're commenting on Reddit:
human
[Hogfather Movie] Are Susan's powers coming from her not-even-really being related to Death? That seems weird enough to me to pose the question: Has she, like every other fantasy creature, just been made up by the kids in her house?
gpt
Susan is literally Death's granddaughter (by adoption) and the daughter of Death's apprentice. She inherited the skeletal hand print on her face, as seen when she blushes, from when her father was slapped by Death.
Her heredity is magical in nature but still there.

Her mother is Death's daughter, even though she was adopted. Susan still inherits from death through her mother because her mother IS Death's daughter. 

Mort is Death's apprentice and receives all of Death's powers and even becomes Death for a short time while performing  the Duty. Susan again inherits Death's  powers from her father he IS Death's apprentice.

Finally, Susan inherits Death's powers because Susan IS Death's  granddaughter. 

Hope that makes sense even though it shouldn't.