Process of elimination: the 2 on the right are not her. They're the only ones we can tell are not wearing mother's earrings.
Far Left, at first, I thought was Itsuki, but looks too much like a blushing bride to be her. But that's not the case! The clues are on covers 7-11. Every one of the girls is holding their finger number with their left hand.
Since it's the left hand: far right: Ichika. Second from the right is Nino. In the middle we have Miku with Yotsuba between Miku and Itsuki.
There you have it. Nagi gave enough clues. I figured how the clues work by process of elimination. We knew the two on the right weren't Yotsuba. He didn't spend all that time on the earrings for nothing. He also made sure that Itsuki had her normal facial expression. Thinking that the Japanese read left to right, and it'd been previously brought up that they had their birth order shown on the covers... and Yotsuba being # 4, the ring finger, that's where she is.
