On the one hand Khushi reminded Arnav of his duties to take care of her until she turns 24, with self-confidence. But she had a fear that Arnav will scold her after she had given utterance to her thoughts and she relieved her tension when she didn't see his presence in her room. It seems her outburst was momentary.
How could Manorama be so heartless towards her niece who lost her parents and has no relatives except her Ashu Bhaiya, Chachaji and Chachi? It's good that Khushi's deceased parents saw their real face and didn't give Khushi's custody to any family members.