Surjit Athwal, 27, originally from Coventry, was killed in India in 1998.
Her mother-in-law Bachan Athwal and husband Sukhdave Athwal, of Hayes, west London, were jailed at the Old Bailey in 2007 for arranging her murder.
Jagdeesh Singh said those paid to murder his sister remain free in India and called for a new investigation.
Surjit Athwal was killed during a trip to the northern Indian state of Punjab in 1998. Her body was never found.
Mr Singh has written a letter to David Cameron to mark what would have been her 42nd birthday.
"He can press the Indian Prime Minister for a full scale investigation into this case," he said.
"Although, there was an investigation conducted by the Punjab police in 2000, there were no convictions for men who killed my sister."
