The incident took place at Adra under Bagnan police station of Howrah district.
The two families, who were distantly related, were not on good terms over some issues and the fruit seller, 25-year-old Sheikh Mojam, allegedly used to threaten the girl with rape, police said.
When the girl's elder brother Sheikh Raju met Mojam on the road today, he protested against those threats and an infuriated Mojam stabbed him with a fruit cutting knife.
The youth was then to Calcutta Medical College and Hospital but it was not yet known whether the knife was brought out there, police said.
Mojam is absconding.
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