I think their relationship was more platonic in XII. The sequel makes them out to be the big brother and sister of the Lowtown Orphans. In Revenant Wings, there's a part mid-game where the party is stuck in a dream-like-world-kinda-but-not-really and Penelo has to think of Vaan in order for them to be reunited. She seems to really come out with alot of feelings for him and does soemthing else that suggests she has affections for him.

In both XII and Revenant Wings, it seems Larsa has a thing for Penelo and Vaan has a thing for Ashe. Balthier also has a scene for Ashe in RW (as well as XII). Not to mention the whole mess with Balthier and Fran, forget love triangle, it's a big web that's never really brought to a conclusion, it's up to future installments to pan it out.