Q] Is this movie a prequel to Street Fighter II V?

A] No, it isn't. The back of the box for some releases of the movie (such as the ones Manga Entertainment did) say that it is, but this movie has no relation to that series at all, and it doesn't make any sense for them to. The characters in this movie are quite clearly older when compared to their II V counterparts, among other things.

Q] Was Shun really Ryu's brother? Is Akuma their father?

A] I don't know the answer to either question. You never find out for sure. In the next movie, Street Fighter Alpha Generations, the "Akuma is Ryu's father" thing is featured, and made more obvious.

Q] Where can I see this?

A] The film has been released on video, DVD and UMD in America and the UK by Manga Entertainment. The DVD version goes for next to nothing these days, alternatively there is also a set out which includes this film, Street Fighter II Movie, Street Fighter Alpha Generations and Street Fighter: Round One, Fight! all together. It was also included in the Street Fighter 25th Anniversary set, which is available for the Playstation 3 and Xbox 360.