Prime Minister Manmohan Singh on Saturday said he was saddened by the decision of his half-brother Daljeet Singh Kohli to join BJP.
"I feel very sad. I have no control. They are all adults," he said while replying to queries on the sidelines of the Padma awards function in New Delhi.
Kohli on Friday joined BJP at a public rally in Amritsar in the presence of the party's prime ministerial candidate Narendra Modi.
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.
Dr Singh on Saturday asserted that Congress could again form the government at the centre.
UPA-III is not impossible, the PM said.
Welcoming Kohli into the BJP fold, Modi had said he will further strengthen the party.
PMO sources had 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 had said, adding the prime minister and Kohli have not been keeping in touch since long.
Prime Minister Singh has six sisters while Kohli is his half-brother, the sources said. The prime minister's mother died at an early age.