I’m getting the feeling that Saito is misunderstanding Akane here, I don’t think her sister is actually dead, since she never said that. She only said that she was “far away” which could be taken metaphorically, but also literally. Only time will tell, but she doesn’t seem to be acting as if she’s dead.
Akane would probably be more honest about that with him, she’d probably even cling to him for support remembering it if that was the case.