having little elasticity; hence easily cracked or fractured or snapped
Although Meg hears her father's words as brittle, she is actually the brittle one, since she is slowly waking into a very cold consciousness after a painful whirlwind tear through a wrinkle in time. The physical coldness suggests the other definition of "brittle" ("lacking warmth and generosity of spirit"), which describes the way Meg talks to her father after she discovers that he'd tessered without Charles.
Her father’s words sounded brittle in her ears, as though they were being chipped out of ice.
wanting in moral strength, courage, or will; having the attributes of man as opposed to e.g. divine beings
The chosen definition fits because Meg's father says he can't compare to divine beings such as Mrs Whatsit, Mrs Who, and Mrs Which, and in the same paragraph, he also refers to God. But the simpler definition of "fallible" ("likely to fail or make errors") also fits because he admits to not knowing where they are since he doesn't tesser very well.
"Formidable" also means "inspiring fear"--this definition would be supported by another description of Mrs Which's voice as "grim" ("harshly uninviting or formidable in manner" or "not to be placated or appeased or moved by entreaty"). Aside from Mrs Which's voice making her "skin prickle into gooseflesh," Meg is faced with the formidable mission of saving her brother from IT and the Black Thing (which nearly killed her).
Mrs Which’s voice rolled formidably across the hall. “Ssilencce, cchilldd!”
be confusing or perplexing to; cause to be unable to think clearly
For ye see your calling, brethren, how that not many wise men after the flesh, not many mighty, not many noble, are called, but God hath chosen the foolish things of the world to confound the wise; and God hath chosen the weak things of the world to confound the things which are mighty.
"Consternation" means "fear resulting from the awareness of danger" or "sudden confusion, amazement or dismay"--for a moment, Meg feels all this as she believes the lie that Mrs Whatsit had sent her to IT to be captured rather than to save Charles. But Meg quickly shakes herself loose and she is again appalled ("struck with disgust or revulsion") by IT.
For an appalling moment Meg believed, and in that moment she felt her brain being gathered up into IT.
"Vulnerable" also means "susceptible to criticism or persuasion or temptation"--this definition is more specific than the chosen definition, because it lists the ways that a person can be wounded (humans and IT have both failed and succeeded in throwing those 3 at Charles). The general definition is a better fit because Meg wants to protect her baby brother from all the possible dangers of the world.
Her own Charles Wallace, the real Charles Wallace, the child for whom she had come back to Camazotz, to IT, the baby who was so much more than she was, and who was yet so utterly vulnerable.