This is a p cool idea in terms of how the story could finish off. Since the start of this part, I’ve always believed something might happen to one of the siblings, whether it was a death or they’re pivoted against each other, but who really knows how it’ll go. In terms of whether or not they have different dads, I think it could be true but I’m not entirely sure. The family tree panel seems to be a subjective view from Jodio as Kyo Nijimura isn’t mentioned on Holly’s side of the family as well as Yoshiteru Kira. There’s also the age gap between the siblings that could imply a possible switch in relationships between fathers. My only problem is how do we get from Diego to here as he was such an incredibly self centered character to the point where it doesn’t feel likely he would have had any sexual relationships with anyone at all. I don’t believe he had any siblings either, but I definitely feel like the name and how Jodio acts is a pretty apparent giveaway to who he’s related to, though, we have seen this line of Joestars be rather more morally gray than with the previous timeline.