So, this birthmark only shows up in the Joestar family, thus Jonathan, Joseph, Holly, Jotaro, Josuke and Jolyne had one.
Since DIO stole Jonathan's body, all of his descendants has a Joestar birthmark as well, them being Giorno, Ungalo, Versus, and Rikiel.
But Enrico Pucci, and Weather Report weren't related to DIO, nor the Joestars, yet both of them posess one. It couldn't be because of DIO's effect on them (like the Green Baby, created from DIO's bone), because while that would explain why Pucci has one, it had nothing to do with Weather.
So, why did they had one?