I feel like the twist is going to be that she's related to Chikage and not the FL. They have the same eye design and it would make sense:
1) Junna's dad talked vaguely about what was probably himself, abandoning his single father and being adopted into his wife's family.That would mean that the Tenkouin line runs through Junna's mom. But it also feels like this isn't meant to be broad public knowledge.
2) Yuki's mom seemed familiar with the dead pianist lady, who seems to be strongly hinted to be Junna's mom, in part hinted at through her musical talent.
3) We learn that Chikage's family serves the Tenkouins for generations and that her mom allegedly died when she was a year old and her family basically doesn't acknowledge her existence for some reason that suggests something bad happened.
So if Yuki's mom was Chikage's mom, she could've been assigned to the pianist lady as a bodyguard/maid and friend. She has Chikage, but then a year after that, in between Chikage being born and Junna being born, something happens and she either runs away or is banished. Chikage's grandfather takes his granddaughter in and acts like his daughter is basically dead and a pariah. Meanwhile she actually sets up a new life, has Yuki, and has dyed her hair so as to differentiate her appearance from before.
The only significant question is how much older Chikage is than the 2 leads in order to map out time for Yuki to be born roughly around the same time as Junna but be far enough from Chikage's birth to have maybe settled into a new life and met someone different (making Chikage and Yuki half siblings rather than full siblings if Chikage is supposed to be particularly close in age to the other two)
I think that making the two leads related pretty much kneecaps the story right from the outset. But making the mom tangential to Junna's family and her story creates drama and adds a layer of character depth to the relationship between Yuki and Chikage.