no. Rachel Robinson and Georgia Van Cuylenburg both said there wasnt anything sexual between Fang and Vanille. they were just really close.
Rachel Robinson claims to think it's open to interpretation in the Project Crystallis interview.
And that's kind of the thing, it's not set in stone, it's open for each person to take it their own way.
And at that, it doesn't matter.
They're clearly closer to each other than anyone else. You can't agree with that. They love each other (what depth of love is irrelevant for the scope of the story, sisters, girlfriends, anything in-between/else you can think of, doesn't change the main fact), they are devoted to each other, in a way that's clearly more than just regular best friends, whether they cross over into lover territory is largely not relevant.
What matters, is they are as close as family, they're intimate enough for it not to be awkward to view each other's thighs up close. That they're willing to become Ragnarok for each other, together with each other. And that they can dream pleasantly together after crystallizing Cocoon.
So it's up for individual interpretation because it doesn't change the story at all. At best(or worst) all it does is add a pink heart shaped frame on some scenes.
The prequel short novel, also showcases their relationship some more. Particularly showing Vanille's just as devoted as Fang.
The best plans are the ones that don't work unless you wing it along the way. -Skandrae Support Project Crystallis! http://projectcrystallis.org/
On another note, they can't be sister, they don't have the same last name. Fang Yun Vanille Dia
They are just both from Oerba, where everyone there lives as one big happy family.
They can have a sisterly relationship without being blood relatives. In one of my recent playthroughs of FF5 Faris and Lenna did seem similar to Fang and Vanille.
As you said, they can have a "sisterly relationship" so they aren't sisters.
But they go over that, I'll never be as close to my brothers and sisters than those two are. It would feel wrong.
They act the same way together than most discreet gay couples do in public, to not bring too much attention. And yet, often the other members of the cast are sorta feeling awkward when these two reunites together, which means something.
Then again, it all depends on how you life experiences affect your comprehension/perception of things.