Well, Fang checked out her thigh once for something very specific (which both sisters and female friends do at times).
But where in the world does the groping each other's boobs incident come from? I swear I don't remember that happening.
You rip up your sister's skirt to check out her butt? Right...
And just look up screenshots of Fang and Vanille to see how they embrace.
If Fang was male, there would be no question about their relationship being romantic.
Truth Bomb 2: Fang was originally designed as male and they changed the gender partway through development to balance the male/female ratios better.
This is like those people who still refuse to accept FFT's ending.
Ramza lived.