Let's have a little fun, with bad fanfiction 8D (but I put some serious thought into it)
I concluded that Sora and Ven must be blood relatives since they both have the exact same face structure, and eyes, and etc. I suspect they both may be half brothers. This would suggest either Sora's mom or Sora's dad has allowed the conception of these two boys. this would also also suggest that Ven is also from Destiny island.
It doesnt really ,matter who it is, but let's just say SOra and Ven both share the same mother. His mom got pregnant when she was really young, like Kairi's age or something. And family problems happened, her boyfriend beats her and Ven up like all the time. Ven had a lot of darkness in his heart. Master Xehanort saw this so he picks him as a potential apprentice. Ven runs away when he''s 14 or some other random age leaving behind the only person he cared about. SOra/Ven's mom is distraught, she breaks up with her boyfriend, marries a redhead guy.
Stuff happens, she got pregnant again when Ven becomes 14.5 or something.
Then kairi arrived to Destiny Islands. Sora's mom enjoys setting up playdates with her and her boy. She takes pity on Kairi for not having parents, so they bonded a bit. Their hearts touch.
Sora's mom says "Dinner time" then she becomes a heartless and bla bla, things turn back to normal again
Sora's mom is also blond, and has Namine's hairstyle except longer.Kairi's nobody takes form looking like Sora's mom because Sora's mom's dna is present inside Sora's body and Ven's heart and Sora's heart influenced Kairi's heart since all three have a place in their heart for Sora's mom which is why Namine looks like Sora's mom
Tho, that would also mean RoxasxNamine is incest but oh well, it's already weird with Xion anyways.