Foreign guests in a Japanese homestay very often run into problems because they misunderstand the family's uchi. They may fail to understand how their "responsibility" to the host family is connected with uchi, and that they, too, need to think of themselves as part of a unit that is "uchi", rather than as individuals steering their own canoes.
Uchi is, in a sense, a "whole" in which the individuals are parts. What the individual says and does is grounded in this relationship. The uchi member is NOT free to "do his or her thing" because each must ALWAYS take into account what he does in relation to the "whole" of the uchi. A rebellious teenager who stays out late is not just overstaying her curfew, but defying the uchi "whole".
"Coming into uchi", then, means becoming a part of the uchi "whole". Sophie violated this in deciding for herself to accept a ride that the okaasan had already turned down. (Here she went against the decision of her family's uchi). Theo's decision to ride the bicycle appeared reasonable from his individual perspective, but his family's uchi perspective was different, and he hadn't been with his family long enough to understand this. Theo thought he was being helpful; once again the problem was timing.