Daniel's sister, Leah, suffered severe mental trauma as a child, and is now agoraphobic (scared of open spaces) and also scared of other people aside from Daniel and their grandmother. Thacia, who becomes Daniel's close friend and love interest, is able to visit Leah because of Thacia's deep love of all people. Leah is not scared of Thacia because Leah can see that Thacia would never harm her, or even consider the possibility; Thacia has room for nothing but love in her heart. This is something that Daniel does not yet understand:
He knew he would never know how she had accomplished it. Girls were strange creatures. He could not understand them. But he could see the change in his sister's face. She was fragile and pale beside Thacia's vivid beauty, but smiling with a smile so like their mother that it caught at his throat.
(Speare, The Bronze Bow, Google Books)
While Leah is also somewhat friendly with a Roman soldier named Marcus, her friendship with Thacia is vital to their personal growth. Thacia knows that Leah needs care to become healthy again, and worries that Daniel cannot provide that care. Leah is scared of others, but welcomes Thacia as a relatable person; both of them are constrained, Thacia by her culture and Leah by her fear, and so they have more in common than Daniel realizes. By allowing Thacia to visit her, Leah shows that her condition is not static, that she can heal, but needs the love of others to reach that healing.
because she trusts her and knows that if she comes and visits her it will be for a while and will always have her back.