If Issa was also living in Tokyo (or at least, had a place to stay given that one office of Ogiwara Foods is in Tokyo), and especially since he was a businessman himself, why didn't he take his own sister in? Surely she would have been better off living with a sibling rather than paving her own way? It's not as if his mother would have to find out about it, he could just hide Sayu if their mother decided to drop in.
Sayu had no way to know that her brother, though he was understanding, wouldn't just hand her back to her mother eventually. And a clean and complete break from her family was what Sayu needed, for her own benefit.
