In The Bronze Bow by Elizabeth George Speare, why is Joel's sister, Malthace, referred to as Thacia later on in the story?

For the first five chapters of the novel The Bronze Bow, Thacia is either referred to by her full name, Malthace, or her nickname, Thace. In Chapter 1, Joel introduces his sister to Daniel as Malthace. Malthace is suspicious of Daniel and is rather distant. In Chapter 2, Daniel tells Joel and Malthace to hide behind a bank of rocks to avoid being seen while Rosh's band attacks a traveling caravan. Joel turns to Malthace and says,

"Thace---you heard him! Get up there---quick---as far as you can...Hurry, Thace!" (Speare 17).

In this situation, Joel refers to his sister informally because Thace is easier to say than Malthace.
Later on in the novel, Daniel visits Joel's home and meets Malthace again. Malthace is rather arrogant towards Daniel and does not go out of her way to talk to him. Joel then tells Daniel,

"Don't mind Thacia...She's putting on city airs lately" (Speare 61).

Thacia is a name that only people very close to her use. Joel is her brother and refers to her as Thacia, while Daniel is a stranger and calls her Malthace. After Malthace cares for Daniel in her home when he escapes from Roman soldiers, she becomes closer to him, and Speare begins to refer to her informally as Thacia. Thacia also takes on a more predominant role in the story, and it is clear that her relationship with Daniel is significant. The change from using Malthace at the beginning of the novel to referring to her as Thacia later on in the story correlates with Thacia's relationship with Daniel and her significance to the plot.