Ibara had thought about Toushiro's question at that time when confronted by him at the dinner table. It was the same question jin said, "Do you want this time when your love for me is shared with others?"
....
Of course, Ibara didn't want to but, she felt sorry for her sister.
Toushiro knows that Guren is now sickly, by working in Tokyo... He will also try to find a way to cure Guren's disease, which is what he told Ibara.
Ibara also doesn't mind if her sister lives with her, as long as her sister and Toushiro are also honest in front of her. Ibara also likewise, he is working in Rakugaki to heal his sister as well, for him it is his love and affection as a brother.
Maybe that's the only way to make up for this unrequited love.
Guren's love is unrequited but, they both intend to repay him by doing something that brings him in a good (noble mind).
