LONDON: The decision by Boris Johnson´s brother to resign from the government has been a particularly heavy blow for the British prime minister, leaving him in a minority within his own family as well as the country´s parliament.
Jo Johnson stepped down on Thursday, saying he could no longer reconcile family loyalties and the national interest.
It rounded off a humiliating week for Johnson in which MPs tore up his hardline Brexit strategy in a series of votes.
He told the BBC he was "very sad" at the news but tried to play it down saying: "Jo and I haven´t seen eye-to-eye for a long time about the UK and the European Union".
In a rambling performance at a press event in Yorkshire later on Thursday, he said Brexit was "an issue that obviously divides families and divides everybody".
Jo Johnson, a former investment banker married to a journalist from the left-wing Guardian newspaper, had already resigned from government under Johnson´s predecessor Theresa May to call for a second referendum.
That left commentators puzzled when, just months later, he joined his brother´s strongly pro-Brexit government.
Brexit has caused severe rifts within many families in Britain ever since the 2016 referendum -- most often been an older pro-Brexit generation and a younger pro-EU one.
That dynamic is reversed in the case of the Johnsons, a highly political family, where the father Stanley has been a strong supporter of staying in the European Union.
A third sibling, Rachel, is also anti-Brexit.
The views of the fourth sibling, Leo, are unknown as he has distanced himself from politics, building a career in the City and as a co-host of a BBC radio series exploring how innovation can transform the way society functions.
Some of Leo´s retweets have raised suspicions, however, including of a comment referring to Brexiteers as "holier than thou so-called upholders of democracy".
Stanley worked for the European Commission in the 1970s and then served as a Conservative MEP, while Rachel ran unsuccessfully for the European Parliament in elections this year for the anti-Brexit Change UK party.
After the news of Jo´s resignation, Rachel tweeted to say that "the family avoids the topic of Brexit, especially at meals, as we don´t want to gang up on the PM!"
The resignation came after 22 MPs left the governing Conservative party this week in protest against the government´s decision to leave the EU on October 31, come what may.
The opposition Labour party seized on his departure.
Deputy leader Tom Watson tweeted: "Once again, the people who trust Boris Johnson least are the ones who know him best."
Pollster Joe Twyman tweeted: "It´s going to be a hell of a Christmas lunch in the Johnson household".
BBC journalist David Cornock quipped: "A rare case of a politician resigning to spend less time with his family".
Friday´s newspapers devoted plenty of column inches to the sibling rivalry.
The Daily Mail ran a headline reading "Et Tu, Jo?", while The Metro front page said "BoJo blow as bro Jo goes".