Actually it was stated by Xemnas, Axel and the reports that Xion is in fact a replica of Roxas, not Sora, that managed to take on Sora's memories through Roxas.
You kinda have that the other way around. Xion is somewhat a replica of Roxas. But she is mainly a failed replica of Sora. The reason she looks like Kairi is because of the fact that Xemnas used Sora's strongest memories. Which, in this case, is memories of Kairi. And I agree, she did have Sora's memories mostly because she was always around Roxas. But she also obtained memories from Sora as well along with having some of Riku's memories.
Now to answer this thread. The reason why Xion looked like Ventus is because that's how Xigbar saw her. Xemnas even stated that he sometimes saw Xion as Ven himself. So it really depends on who looks at her.
When did Xemnas said he saw Ven in Xion?