What a disjointed manga. What even is the point of the 2nd girl anymore. It's like an editor wanted to make a love triangle with sisters and the mangaka agreed to the premise and then said fuck it and went all in on his favorite girl.
It would have made more sense if there were more Yotsuba lovey dovey scenes but even though it was kinda obvious she was the girl, there wasn't really any chemistry between them.
I called it. All the chapters leading up to the scene where they were on the swings pointed to Yotsuba. I even commented on that chapter that it was her.