Yeah, I agree with the sister interpretation. I think this little story arc also allowed Shion to think about his feelings towards Nezumi and realize how deeply their meeting 4 years ago had impacted him.
As for Safu - I had only really gone as far as thinking she was being calm about the possibility of him liking someone else, rather than having completely accepted it. Thinking about it now though, I can see how it might be the case. But even if she has accepted it, I think she’s (understandably) disappointed and will need some time to move on. I guess I’m mostly hung up on how best to interpret these lines:
“Saying I don’t mind if there’s someone else you like would be a bit of a bluff. Pretending that I’m fine no matter what happens is a bad habit of mine.” (roughly)
Plus, she ends the call abruptly by reminding him how she feels. This kinda seems like she might still be hoping Shion will eventually come around, but it could be something else.