A wonderful example of this is in episode 6 of NEXT, when Lina and Amelia were in the hot springs, and Amelia was washing Lina's back. They were talking the way a pair of sister's would:
"Despite all the traveling we've done, Lina-san, you're skin's still so smooth and clear!"
"I guess I'm just built differently than other people."
"True, you're definantly not the delicate type."
"WHAT WAS THAT?!" (kinda reminds me of a conversation my sister and I had once, minus the back scrubbing, that is. ^^;;)
Shortly after the shot of Zel cutting his hair, we hear vigorous splashing and laughing from the girls side, and then it cuts to Lina and Amelia who are having a blast throwing water at each other. They also argue like sisters in episode 3 (I think) of NEXT (the restaurant incident).
In the beginning of NEXT, Lina saved Amelia from Xoana, even though she claims it was because of the Xoana book. Makes sense to go out of your way to save Amelia when you already have the Book of Xoana in your hand, right? According to some of the other pages I've seen, Lina would have just snagged the book and left Amelia to rot because she "hates her guts", but that's not what she did. She also went out of her way to try and cheer Amelia up when it was believed that Phil was dead, and she also protected Amelia when Zel came barreling through the door tossing a Flare Arrow at that guy in the window. Not only that, but she agrees to follow Amelia outside of the palace in order to track down clues towards the death of her father, even though she personally thinks it's a very dangerous idea.
These two are very formidable when they decide to agree on something. In the Lake Dragon episode of NEXT, Lina is bound and determined to use Zel as their anchor. Zel appeals to Amelia to talk to Lina, because he knows that Amelia is the only one Lina would listen to at that moment. Instead of helping, however, Amelia stuffs a hose in Zel's mouth and a funnel attached to it, and they toss him overboard. makes me think twice about what would happen if they teamed up all the time. ^^
Lina cares about Amelia, just like she cares about all of her friends, but she doesn't show it the usual way, because she doesn't like to get embarrassed, and revealing your feelings for others can sometimes be very embarrassing. So, she shows her feelings other ways, by watching out for her friends and protecting them when she sees that they're in over her head. More to come later, as always. ^^