Okay, so here's how it went down.
Mitsuki and Nanaka have a thing for each other, right? They keep that "thing" going for a little while until Hiroshi finds out, in which Nanaka promises Hiroshi that Mitsuki meant nothing to her. It took some time, but when Nana-chan was faced with losing Hiroshi, she decided that she loved him more.
Mitsuki becomes heart-broken, and leaves. Yes, that's right, she leaves. It takes a few years, but Nanaka actually finds out eventually where she left to. By the time Nana-chan finds out, Hiroshi and her have already broken up. The child stays in Nana's care, and Mitsuki and Nanaka raise it together.
John actually dies... Satsuki is heart-broken as well. During the time that Nanaka and Hiroshi were together, Satsuki stayed with Mitsuki. However, when Nanaka and Hiroshi broke up, Satsuki was the one to comfort him. Because of this, the two fall in love and raise Satsuki's child.
Mitsuki's little brother grows up a bit, and decides to live with his big sister and Nanaka. Together, Mitsuki and Nanaka raise both their child and the little brother.
And this is the lovely tale of the big family that came together in such strange ways.
Note from the author: I originally intended to write a lot more for this story; to surpass my Peach Girl story. However, time passed and it grew stale. No one read my story and I had little to no inspiration to keep writing it. I didn't want to leave it at the way it was, though it was somewhat conclusive already, so I summarized the ending. Hope this doesn't bother you.