1:Y Rani ma tie Bari Rani Ma in Secret room?
I don't think Rani Ma is related to Badi Rani Ma, she most probably working for the King in the Palace and so is Daima, and she had access to all the papers,when he died ,she took over as Rani Ma and hide away Badi Rani Ma, have no choice to adopt Yash ,because Monster, Jyotika don't recall any unty, so they must have been very small when it happen,or not even staying at the Palace then.
Yash must have been with Rani ma from young age see her everyday at the Palace,therefore he did not miss his mum.
2:Badi Rani maa definetely.
3:what will be Yash reaction after knowing the truth?
He will be very hurt and heartbroken because he respect and love Rani Maa, but he will be more upset for his mum , the way she has been kept all these years without his knowledge. and will do his utmost to regain what rightfully hers, i.e her title as Badi Queen.
4:how Reva will save Bari Rani Ma from Rani Ma and Daima?
She will try to find out everything from Badi ma how it happen then she might use some sort of Blackmail tactic with Daima,because Daima is more scare of Yash now that Rani Ma, so Daima will pacify Rani Ma to change her mind about harming Badi Rani maa .Until she can convince Yash may be with the help of Badi maa about the truth.