What exactly is Da Capo if about? Also Kotori and Jun'ichi's granddaughter?

I've watched Da Capo and Da Capo S.S. then right after i watched Da Capo if and i was wondering if its about what would of happened if Nemu really died at the end of Da Capo. Also in Da Capo II there is Nanaka Shirakawa who is Kotori Shirakawa's granddaughter from Jun'ichi and i just also want to clarify what that means.