Laxmi, resident of east Delhi's Mandawali area, was repeatedly knifed by her brother Sujit, 18, around 1 a.m. Tuesday.
She was rushed to the Lal Bahadur Shastri Hospital where doctors pronounced her dead.
"Both the siblings study in a Sarvodaya Bal Vidyalaya in Class 11. The girl had an affair with a boy. Sujit told Laxmi to break up her relation with that boy but she continued," said a police officer.
"Last (Monday) night the boy stabbed his sister several times and fled from the spot. A case has been registered and hunt is now on for Sujit," added the officer.