This episode occurs in Chapter 6, and is when Daniel has to find sanctuary very quickly after he insults a Roman soldier. He runs to the only place that he can go to in order to find sanctuary, which is Joel's house. However, Joel is not there, and it is only Malthace who is present, and she makes it very clear that Daniel is not welcome, as he will bring trouble into Joel's life. Note what Malthace says to Daniel when he tells her he must speak to Joel:

Nothing is so important as Joel's studies... If you cared anything about Joel, you woul dleave him alone. He can be a famous rabbi someday. He's not going to risk his whole future for a band of outlaws.

Malthace therefore refuses to greet Daniel hospitably because she fears her brother's attraction towards Daniel's revolutionary cause and ideals, and doesn't want her brother to jeopardise his future and potentially become involved in something that would place him in danger. It is only Daniel's sickness that causes him to stay in Joel's house and allows him to develop his relationship with Joel further.

Why does Thacia refuse to greet Daniel?

Thacia’s reaction to Daniel when he enters her home characterizes her as a very different sort of person than the one he first sees on the mountain. There he notices “the way that her hair had sprung, clean and alive and shining, like a bird’s wing, aback from the smooth forehead.” Her effect on him is temporarily profound, for the “grace” he sees in her reminds him of Leah, the thought of which “stirred in him an old wound” (24). Thacia’s “shrinking” from Daniel when he enters her house corresponds to Leah doing the same when he first returns home to Leah. While Thacia is, as Josh explains, “putting on city airs,” Leah has been deeply traumatized, but in both cases Daniel must in some way prove himself to these women to garner their affection and respect. Daniel does this with Thacia when he returns for help after he is wounded, demonstrating a form of bravery, devotion, and vulnerability that she admires, and he does this with Leah when he cares for her day after day, indicating he will not abandon her again.  In the first case, the girl cares for the boy, reminding him (in his delirium) of his mother, in the other case the boy cares for the girl, providing the nurturance of a mother.

Thacia is going through a bit of an identity crisis.  She has been raised in a strictly observant, wealthy Jewish home, one that holds some negative opinions about classes lower than own.  When Daniel arrives at her doorstep, bloody and filthy, she is temporarily at odds with her family's values and her own sense of morality.  Although at first she is rather cold to Daniel, soon she realizes the error of her ways and becomes the best friend Daniel could ever have and the most loyal companion one could ever want.