I couldn't help but notice that Charlotte is very cold and sarcastic to almost everyone except Daniel, who is in many ways the most fragile. She really watches out for him. Like a big sister or maybe it has (or will) developed into something more?

__________________
"Helen, do you remember that authentic Australian walkabout I told you about?"

I thought the same thing - tough, older sister protects (and gets done what is necessary) for the younger, fragile-genius brother.

__________________

`Dear, dear! How queer everything is to-day! And yesterday things went on just as usual. I wonder if I've been changed in the night? Let me think: was I the same when I got up this morning? I almost think I can remember feeling a little different. But if I'm not the same, the next question is, Who in the world am I? Ah, that's the great puzzle!'

-- Alice, from “Alice’s Adventure’s in Wonderland”

I didnt know they were sister and brother?

No, we're saying the relationship is like that of a brother and sister. Not that they are actually related. But with Lost you never know.

__________________
"Helen, do you remember that authentic Australian walkabout I told you about?"

Got ya! LOL! I thought I missed something for a second!

These two have both attended Oxford. Similar age, maybe they are/were friends and co-workers. Who knows, that trophy that Charlotte holds in the famous Abaddon files photo might be for something she acheived with Daniel, or via his help.

The way they act towards each other, makes me wonder, they both know something, and are very close I must say!

I too think that Charolette treats Daniel like a little brother.. It seems that she feels she needs to watch out for him... Much like an over protective big sister..

I was thinking she seems to protect him from himself.. No Daniel don't say that.. Just do what I said Daniel. You get the idea

__________________
Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.