Many of us do not like Sia's excessively mature (I doubt though), cute (again I doubt 😃) and beautifully emoted (now no comments 😉) character.
I remember when Sia was throwing herself on Abhi, and he was stunned, she had a slap from him, we may not have heard the sound but when the truth was revealed, it was more than slap.
Sia's Simran'ish nature is the root cause of Astha's problems. Since Sia came into story, she caused so much pain directly or indirectly to Abhi and Simran's family. She made it too late to inform her parents about Astha's connection with Kiran that the situation went out of control. After failing in her first love venture, she conveniently turned to Astha's love.