Spoiler warning to whomever may be reading this.
A bit before Stardust crusaders, Joseph met Tomoko (Josuke's Mom) and they had a bit of an affair, which led to Josuke, and since this is well after, Joseph and Suzi Q had Holy, like, 41 years after Holy was born. (Assuming my math was right.) and due to this, Holy and Josuke are half-siblings, making Jotaro, Josuke's half-nephew, regardless of age difference.
Though, Suzi Q still forgave Joseph and they're still together. and alive surprisingly. (in their universe that is.)