The entry of Kohli, a local businessman, into the BJP came a day after the prime minister rejected perceptions of a 'Modi wave' in the country. Lok Sabha elections in Punjab are due in one phase on April 30.
Welcoming Kohli into the BJP fold, Modi said he will further strengthen the party.
"Today, Prime Minister Manmohan Singh's brother Daljeet Singh has joined BJP. This will further strengthen us. We are not a party which talks of membership but one which forms relations," the Gujarat Chief Minister said.
PMO sources said Manmohan Singh's family was "shocked" at the turn of events involving Kohli. "His intentions are not known but he is free to pursue his political career," they said, adding the prime minister and Kohli have not been keeping in touch since long.
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Prime Minister Singh is the only son and has six sisters while Kohli is his step-brother, the sources said. The prime minister's mother died at an early age.
Singh yesterday dismissed that there was any 'Modi wave' in the country and claimed it was a creation of the media.