Considering this curse has never been completed before we can determine that the ancestors would have died early so there is a good chance they didnt have time to procreate. Therefore, the cursed descendants are most likely descended from the ancestors' relatives.
That being said it is not impossible that the two originals had offspring somehow but I doubt it since the curse would have caused them to die early.
Yeah, and there's also that weird person Byakuya was talking to before (in ch 16). Either that was the grandma that was taking care of him or someone else. But if it was the grandma, why would the author not show her face??? I wonder if that person is related to the curses .-.
I'm still in Chapter 19, so I don't really know much. But... Why do I get the feeling that their ancestors didn't end up together but had separate families of their own??? And that's the reason there's a curse??? OMG.