The clue for mistaken identity comes from the episode when Ayaan had to identify Humeira's body. He had thought that it was Humeira based on the ring found on the dead girl's finger.
My take on this that Humeira and this girl (Rajini Shukla) got to start talking on the bus. Rajini Shukla would have shared of her husband Vikram Shukla and how much he loves her and maybe the story of how they met giving Humeira a perfect of love of which she had always been dreaming of with Ayaan. Rajini would have admired Humeira's ring and asked if she could try it on and Humeira would have given it to her. Rajini Shukla could have also been the bubbly kind of girl that Ayaan likes and that would have also crossed her mind. Humeira would have wished that she was more like Rajini's and maybe then Ayaan would have loved and this got embedded in her subconcious mind. That is when the bus accident happened and the last thought in Humeira's mind would have been Rajini Shukla and her story and the perfect love and wished probably that she could have taken her place.
So when Humeira regained her conciousness, her subconcious mind would have taken over and she would have shut out Humeira and taken over Rajini... There is a medical term for this kind of amnesia, (Identity Amensia (????) which is supposed to be temporary.
Sometimes, however, it is their only their personality that has changed. they are convinced that they are someone else entirely. The affected person may have additional amnesia on top of their personality change. Humeira has gone through a personality change and has shut out her previous life for Ayaan.
In one of the previous episodes, the fashion show... Humeira took part in the fashion show for Ayaan trying to be the kind of girl he likes...
Humeira had led a very sheltered life and Ayaan's rejection was very truamatic for her and she must have shut out her self and everybody out.
Makes sense?
I think in the Sidiqqui house, we are going to see some fun times, AyRa romance, Humeira or Rajini is going to find Badi Bhi before she dies and she maybe the one who also literally gets Razia kicked out of the house.