I'm not much for shipping--I generally take whatever pair is handed to me and just accept it--but I have to agree, Larsa/Penelo is adorable, and superior to Vaan/Penelo in so many ways. Firstly, Penelo really did seem like Vaan's caretaker, or older sister, and not a romantic interest. Clearly he's too immature for her, and I imagine that any relationship they could have would be comprised of her looking after him and him doing brash and inconsiderate things in the typical, "i'm an obnoxious video game hero" sort of way.

Larsa, on the otherhand, is incredibly mature for his age and he's still growing, so I hold hope that he'll begin to look a little less feminine as he ages and maybe inherit some of his brother's masculine features. Aside from the cuteness between Larsa and Penelo(and the ineptness of Vaan) in the OP's clip, I love Ashe's reaction to Larsa's mini-speech about doing what is best for Dalmasca and the Empire. There is this slight moment of hesitation before she hesitantly responds, "very well, then" where I'm sure she's thinking,
"HOW old is this kid?"

On a side-note, I don't really think of Penelo and Larsa's hypothetical(possibly fannon?) relationship as Poor Peasent Girl/Noble, because I don't think that would be the dynamic of their relationship. Others might view it that way, but I'm sure there would be very little distinction made on either of their parts. Their personalities seem more compatable than Vaan and Penelo's as well. Both are cool headed and gentle-hearted, and I always thought that Penelo might be smarter than the game's creators let on. Darn you for poor character developmen SquareEnix! That's okay--we have fanfiction! Fanfiction fixes everything! Muahahaha!