Also, in the precap Suhana reacted that way because she doesn't know that Devakshi are Ayush's parents. This is something that she needed to be prepared for before breaking it to her.
Dev told Suhana that Ayush was his friend's son. I don't understand why they didn't tell her that he is her brother? Before bringing Ayush into their home, they should have sat down with Suhana & tell her that she has a brother whom they had lost long ago but thankfully found him now. So they will be bringing him home soon. Who knows she might have been delighted at the prospect of having another brother? Even this new Soha still loves Golu & Shubh. She's not reluctant to accept a brother, she is reluctant to share her things with someone who is practically a stranger, doesn't belong here (from her point of view) & still being treated better than her.
Right now for her, it's like a strange kid came, who is not related to her or her family anyway & still she is being asked to share her room, her books, her toys etc with him. And now she can see that he is going to share her parents too.
This is a total mess & I blame Ishwari for creating this mess & Dev for not stopping her. They should have put Ayush in another room, got him new furniture & other stuffs like Sonakshi suggested. Ayush himself is not comfortable in this house, it would have been relaxing for him too if he had his own space. But Ishwari, the moment she heard Ayush is potentially her real pota, started trying to fit him in into the place that had been Suhana's for all these years. Suhana just came back from school one fine day & found out that her room is not her alone anymore, neither are her toys & other stuffs. And now again, it's Ishwari who jumped in & announced that Ayush's parents were Dev & Sonakshi, in front of Suhana, when she had no clue about it. Ishwari might not want to evict her right away, but it's very evident that she has totally stopped caring for Suhana. She is not deliberately hostile towards her, not yet, but doesn't spare a thought over how is she doing anymore.