SPOILER. The group will eventually find the siblings as shown in the Gakuen Alice Memorial Book.
However, clues of their current whereabouts were scattered in Kageki no Kuni no Alice.
Clue 1: The dream of Imai Shizuku. Prior to entering the Music School, she kept dreaming of a girl named “Imai Hotaru” that used to study in the academy. She later joined the music school to unravel the mysteries behind her dreams.
What is Shizuku’s role in the story? I think she will serve as a medium between the present and the time-space world. I don't think that she is the grown-up Hotaru despite having the same mannerism and personalities because she said that she and Hotaru are relatives. Also, Shizuku's character design is different from Hotaru's character design in the Gakuen Alice Memory Book.
Clue 2: The place where she gets the Alice Tools in the academy. Could it be a portal? Can the group use this to constantly communicate with the siblings? Who knows?! ^^
Clue 3: Hikari's role in finding Imai's. Recall that Natsume helped in testing Hikari's potential and resolution for joining Alice Revue Company. When Narumi thanked him, he said "Well, we need her to find Imai after all”.' Hikari is a non-Alice so how can she help? She is valuable to the Rei brothers so I’m thinking that maybe both Ema and Aruma are needed in finding them.
Clue 4: The Succession Necklace. The Alice Stone Necklace is proof of the throne successor in Ema and Aruma's Homeland (Sukoku). The stone may possess 3 powers - a barrier, wish, and telepathy alice. The power of the necklace that Hikari now possess may be important in locating the Imai siblings in the Time-space.