@Di3GO95 it’s the author’s way of misleading the readers. Not the first time and not even a rare thing in his manga. She basically got trashed by Futarou so this is her way of saying “Alright, you win just this time” to Miku.
Edit: If you compared the page she smiled at and the page before it (where there’s only a panel of her lips smiling) it’s obvious what the author wants to convey. First, reveal that Ichika got hold of an important piece of info to group with Futarou. Next, show an evil smile in that small panel. Then next page, a full panel of her smiling. She could be scheming, or smiling in acknowledgement of her sisters.
Of course, this requires context, which you will get fast-forward a few pages where it is announced that she is in group D instead. Now that full-panel smile makes sense: she was smiling as a big sister setting up chances for Miku. It’s her way of making up to all she has done to Miku and also a nod of defeat, albeit only this event.