Actually rather than a spoilt brat she looked more of a hurt child who is still unable to accept the new person that is being forced into her space and life. Also the differences in the reaction kind of justifies Sonakshi's behavior and fear, when Ayush breaks a glass Dev, Ishwari and even Sonakshi's came down to check on him, while Dev and Ishwari were comforting him, but when Suhana broke a glass no one except for Sonakshi herself bothered to check on her, so how can Sonakshi not feel insecure about Suhana and alienated towards Ayush? For her Suhana is only her daughter, deep down she probably has a feeling like a hunch that probably Suhana is not adopted but her own and she can't stand watching her child be treated like an outsider or some secondary option.
It cements the thinking of each camp. Sona’s fear about soha being left out in the cold and Dev and Ishwari about Sona not accepting Ayush whole heartedly. The way they showed the flower crushing was nothing short of vampish behaviour. If any child behaves likes this the audience will clearly sode with poor Ayush who seems like the more bechara in this situation. Absolutely agree with you.