Boris Johnson’s Brexit-Fueled Family Feud

While the incoming U.K. prime minister cheerleads an exit from the EU, his father and siblings have other ideas.

Surrounding Boris Johnson, Brexit’s highest-profile supporter and Britain’s next prime minister, is a group of people who really don’t want to leave the European Union. Awkwardly for Mr. Johnson, they are his family.

In the three years since Britain voted to leave the EU, Mr. Johnson has been dogged by this crew of dissenters. His sister took her top off on television to promote staying in the EU. His youngest brother quit his job in government to promote halting the Brexit process.

Even his largely supportive father, Stanley, got in on the act, penning a fictional thriller about shady politics ahead of a referendum to leave the EU, titled “Kompromat.”

When he becomes prime minister on Wednesday, Mr. Johnson will surround himself with a mix of advisers who both supported and opposed Brexit. But Mr. Johnson is known as something of a political loner who for years dispensed with a large Westminster entourage. One person who attended Mr. Johnson’s 50th birthday party five years ago was amazed at how few politicians were there.

Mr. Johnson has remained close to his family,

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