This is actually a pretty good theory. Somehow I don't think it's something Nomura would pull out, but it's possible. People's memories have been altered in Sora's case, so it's not like Ven couldn't be erased from everyone's memories. If Sora was born after Ven went missing and his parents forgot, it would explain why Sora doesn't know about Ven. It might also explain why their names are similar in meaning. If Sora's parents named Ven something that has a definition related to the sky, they might subconsciously repeat that in a different language after losing their memories. After all, Namine said memories aren't really forgotten, they're just taken off of the "chain" and buried under the newly created ones. Although the flaw in that is that Aqua's and Terra's names are similar to Kairi's and Riku's respectively.
Then again, Terra was watching who it has been suggested were Sora and Riku playing on the beach. So that basically proves that it's not so far back Sora wasn't born yet. Perhaps he was doing it as a request from Ven, who for some reason still remembers his family, even if they don't remember him.
I still find it unlikely, but it's a really good theory. Kudos to your brother.