Rachel Johnson, the sister of Tory Boris, is to stand for Change UK in June's European elections.
Also standing for the party formerly known as the Independent Group will be Gavin Esler blasts 'worldwide joke' Brexit UK as ex-BBC man moves to politics and ex Tory MP Stephen Dorrell.
Ms Johnson, who has long campaigned against Brexit, has become the second sister of a prominent Brexiteer to join a new party in as many weeks.
Jacob Rees-Mogg's sister Annunziata announced last week that she would stand in the election for Nigel Farage's Brexit Party.
Announcing her candidacy in the Evening Standard, Ms Johnson took a swipe at her brother, saying she did not want to see Brexit “rubbing out my children’s prospects and chances of living and travelling and working in Europe”.
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She said: “These are chances that the politicians who decided to campaign to Leave have enjoyed themselves.
“It is simply not fair — and sometimes one has to stand up and be counted. It is now that time for me.”
It came as the new anti-Brexit party - formed after 11 MPs quit to form The Independent Group in February - unveiled its candidates for the May 23 EU elections at a launch in Bristol this morning.
Interim leader Heidi Allen claimed 3,700 people offered to stand - and said the candidates would be "standing up and speaking up for the mainstream Britain we all know.
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She claimed "the call went out from TIGger towers" and she was a "proud TIGger mum".
"If you're fed up with our broken politics, join us, get involved and change it forever", she said.
"We demand a people's vote and the right to campaign to remain in the EU."
Former BBC Newsnight presenter Mr Esler said: "Our political system is a joke. It's a worldwide joke."
But Ms Allen earlier faced criticism for saying she will not force a general election to change the UK government.
Heidi Allen - the ex-Tory MP who has taken the helm of the former 'Independent Group' - signalled she was unlikely to back a no confidence vote in the Tory government.
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And she confirmed: "We won't precipitate a general election."
Change UK is the official name for the Independent Group, which was founded by 8 Labour defectors and three Tory defector MPs in February.
The MPs quit largely over frustration with their parties for refusing to whole-heartedly oppose Brexit.
But they have refused to face by-elections after changing their allegiance and say a general election would not be the right response.
A general election is triggered automatically after 14 days if the sitting government loses a vote of no confidence.
But for one to pass it would need the backing of key MPs, such as the DUP group who back Theresa May or Change UK.
