Did you know that Fang used to be a man in the original plot?
Square-Enix changed thier mind on Fang's gender during the middle of the game's development. Due to thier deadline, they did not have enough time to change the scenes between Vanille and Fang. They only changed the model of Fang's and some dialogues.
Source:Originally Posted by IGN
Final Fantasy XIII Update - PlayStation 3 News at IGN
Now, you should start visualising Fang as a man and then re-think the whole storyline. Suspect anything?
With Fang being a man, I think that everything between him and Vanille would make more sense. Consider:
(SPOILER)Vanille touching Fang's boobs
(SPOILER)Fang pulling Vanille's skirt
(SPOILER)Calling her a partner
and more.
The point of this thread is that if Fang was still a man, you all would automatically assume that Fang (male) and Vanille are in a relationship. Just like Snow / Serah. Lol, typical ...
But Fang is a female now, and you all assume that they are not lesbians, just sisters. Why? There's nothing wrong with lesbians, you know. They don't explicitly say 'I love you' to each other, why? It's just how the Japanese culture works.
Conclusion: they changed Fang's gender while the importance of between Fang and Vanille remains the same due to SE not having enough time to re-change everything. This proves that Vanille and Fang have a strong bond with each other. They are lebsians.
You know... if you actually consider Fang and Vanille as sisters, what about Lightning and Hope? Lightning and Hope are obviously brothers/sisters to each other. But yet they don't have that strong bond like the one Vanille and Fang have.