Dinah is a descendant of a member of the lynch mob that killed Hetka
Lots of waffling, so please bear with me. We know that Dinah currently lives in Bizenghast because her aunt inherited St Lymans. How did Dinah's family end up owning the school in the first place? My theory is that Dinah is related to one of Bizenghast's old families, probably the Rosacruxes. The patriarch, Jeriah Rosacrux believed Bizenghast had become a village of sin, and was actively trying to correct that, so he likely organised Hetka's lynching and was present at the event. After St Lymans burned down, one of the old families probably bought the place to keep outsiders from finding Addie Clark's remains
, and then left it until Dinah moved in.
Vincent is related to Rosemary from "The Plague Bearer"
The family resemblance is