I really do like this series so far. It's realistic (this has happened to many a teenage girl), the characters are outspoken and mature when they are expected to be, and it makes me wonder, due to the title, where it'll go.
Option 1 is she moves on, and it's a short story.
Option 2 is a time skip where sister and Midori separate and Sui and him reconnect later in life.
Option 3 there is no option 3 she's like 14.