Rachel Johnson has said she has yet to meet her brother Boris's partner Carrie Symonds.
The journalist and broadcaster made the revelation on ITV's Good Morning Britain after Ms Symonds was hailed by host Piers Morgan for being "a bit of a star" for the Conservatives throughout their General Election campaign.
Ms Johnson said: "I haven't met Carrie. Not yet."
When met with disbelief from the presenter, she added: "It's not something I came to talk about."
Their father Stanley, who appeared alongside Ms Johnson on the programme, said that he has met Ms Symonds, adding: "The Johnsons are united in this."
The claim from Ms Johnson came as her brother and his girlfriend arrived at Number 10 Downing Street after the Conservatives returned to power following the General Election.
The couple waved at photographers after scoring the biggest Conservative majority since the 1980s.
Mr Johnson, 55, and former Tory spin doctor Ms Symonds, 31, are the first unmarried couple to officially live in Number 10.
Her association with Mr Johnson dates back to early in her career, having worked on his successful re-election bid at City Hall in 2012.
Mr Johnson's marriage, to second wife Marina Wheeler, ended after 25 years together last year.