I can already see.
The girls were sad because their mother died, left them or something, Papa took them somewhere for a change, one of them met Fuutarou and they became friends.
She then talks about their situation and how she wants their sisters to go back to their old selves.
Fuutarou talks about how she should change if she wants the others to change too. Yadda yadda pic happens.
Even without all this shitpost of mine, the only two girls that could be the girl is Itsuki and Yotsuba anyway.
Ichika is too lazy and goes "my pace" too much to be an exemple to everyone.
Nino isnt trying to be an exemple, she's just over protective of her sisters.
Miku is gloomy and anti-social.
Yotsuba is active, positive and always trying to set a good mood when she's around.
Itsuki tries hard and her best, and takes things seriously, she just fails because she's dumb.
Conclusion is, chances to be the girl from the past
Itsuki = Yotsuba > Nino > Ichika >= Miku