At the time Sia realizes that Singhania(Karan) is not what he claims to be but he's her ex Vikram she totally ignores the height difference Vikram was about 6 feet tall and Karan is 5'9".
In movie, Vikram Diwan realizes that the bomb was planted by his younger brother Aditya Diwan and his girlfriend Siya.
No one told him this and he can not blindly say to his friend Karan in the hospital that they both have planned to murder him.