Blame it on the script writers of Kyunki who keep reusing this silly concept of someone longing after their brothers/sisters love interest.
The entire Virani Khandaan has it in its blood to do this. So many of the love triangles were like that
Karan-Gautam-Damini
Karan-Nandini-Ansh
Sahil-Ganga-Gautam
then you have
Bhoomi-Archita-Abhir and KT-Laksh-Eklavya.
Wonder when its gonna stop.