They still hasn't got back into the boyfriend and girlfriend status. After 4 months, they even starting to get comfortable with the weird distancing of being siblings with history.
They both still have regrets from back then though, such as, what if they did things differently, what if they try to fix their relationship. Even as they think of letting go the past, "love just wont give up".
Mizuto is actually a detached person. Because of his upbringing (his mother died after giving birth, so he never experienced that part of life) he sees the world as an observer. It's not until their first date, when Yume got lost and cried to him from the phone (this is at the end of volume 1, so probably close to chapter 18~20 of the manga), that he consider her to be special, the only real person in his world. Yes, she's that special for him. But he probably want to keep their current distance as siblings, avoiding the "lovers" label that had torn them apart before.
Yume is also still caught in the past, but after volume 4 she finally decides to move forward. She admit, she still love him, but will no longer be trapped in the past. She wants to win his heart again, and this time against her old self who he still can't let go.
There is another girl, Isana, a loner light novel otaku girl. Cute, big boobs, living life at her own pace, Yurucamp's Nadeshiko's voice. Even Yume think she's a perfect girlfriend for Mizuto. He also admits, Yume's victory is because she came to his life earlier than Isana.
But she's rejected when she confessed (Mizuto still loves his old girlfriend) so she returns to being a very close friend (described as "couple who has been living together for 3 years and wouldn't be surprising if one of them suddenly says 'Hey, lets get married' out of the blue", or between a big dog and her owner, even his parents thought she's his ex-girlfriend) and is actually enjoying the one-sided love more than if she actually become the girlfriend, so she's not in the competition.