(Note: this isn't based on any current factual knowledge, It's just a guess.) I think that it is highly probable that Ven could possibly be Sora's brother. I mean the age difference completely eliminates the possibility of Ven being Sora's father or anything like that, so would it not be possible for him to be Sora's brother? Just take a look at what Nomura has said about Ven and about some of the scenes we have seen in BBS.
First off, I wondered ever since I saw the scene with Terra on the beach of Destiny Islands, why would he be there watching Sora and Riku in the first place? Then after I thought of Ven being Sora's brother I said well maybe that is something Terra does for Ven. Maybe he keeps an eye on Sora for Ventus, espescially considering Ven apparently saved Terra at some point. Now I also know you may think, "Well why doesn't Sora ever at any point say something about a brother?" The answer to that is more than likely Ven left or was taken on as an apprentice to TAV's master before Sora ever knew Ven.
If Ven was Sora's brother, that would also eliminate any connection between him and Roxas other than the fact that they look alike, and Sora wouldn't know because he never met or knew he had a brother. Like I stated at the start of this post though, it's just a possibility and has no factual information to back it up, so take as you like. I just think it has a good chance of being correct.