I don't think Claire is so much crazy as she is confused. As far as we're aware, she suffered a concussion back when her house at the Barracks was attacked, and the aftermath of that showed she was really out of it for the next few hours or days, at least, which was when she left Aaron behind and followed Christian Sheppard.
It's very conceivable, especially considering the twists we saw with Rousseau, that Claire has minor amnesia, and cannot remember exactly what happened during that time, even forgetting what happened to Aaron. Furthermore, from this perspective, Claire really isn't all that crazy. She's been abducted by the others before, and it when the rest of the Lost crew left or time-traveled, it's not like the Others were friends. Alone, she'd have no choice but to consider them enemies, and I can see how she would believe they took Aaron, as well, given they did want to take him in the first three seasons, anyways.
My real question is in regard to what Christian Sheppard and unLocke told her... Did they say the Others took Aaron? Were they both there at the same time? Why would they lie to Claire? Could it actually have been to protect her from them, since, as far as we can see, they do want her dead, and they're not exactly great people, still.
What's important to remember is this: It seems the Man In Black, aka unLocke, aka the Smoke Monster is stuck either in his "pillar of smoke" form or as Locke, but Christian Sheppard actually appeared last season after we saw unLocke. If we believe Ilanna, it means Christian Sheppard is a separate entity on the island. My honest guess? He exists as a balancing force of some sort. A referee, if you will. Recall Christian Sheppard's main role has been to take people where they "need to be," in a sense. In season one, he guided Jack to a fresh water source for the island's survivors. In season four, he did take Claire to Jacob's Cabin, and in season five, he showed Sun and Frank Lapidus that the rest of the Losties were in 1977. While it seems like he helps the Man In Black more often than Jacob, I believe he only does so because he'd deemed "The Others," and similar forces an unfair advantage in Jacob's favor, in regards to whatever game the two have been playing.
That's just the idea I've been formulating from what I've seen, and it explains why he's flat-out helped the Losties, while the smoke monster was generally a nuisance.