Annunziata Rees-Mogg said her brother Jacob would be “devastated” after the Brexit Party won three European seats in the East Midlands.
After the result was announced in Kettering, the newly-elected MEP, whose party won 38.23% of the vote, said the Conservatives were paying for the Theresa May “betraying” the British people.
The Liberal Democrats also won a seat alongside Labour but the Tories returned no MEPs after polling just 10.66% of the vote.
Ms Rees-Mogg described the the results as “quite astounding”.
She said: “They have been far bigger percentages than I had anticipated, although while I was on the streets in areas as diverse as Boston, Mansfield, Skegness and Chesterfield, the people were telling us they wanted to be listened to.
“Today, they have had their voices heard and we will fight for them tooth and nail.”
Addressing how her brother will be feeling after a disappointing evening for the Conservatives, Ms Rees-Mogg said: “I have no doubt he will be devastated at what has been done to his own party, the Conservatives.
“Theresa May has betrayed the British people. She made a lot of promises she didn’t stick to and unfortunately the whole party is paying for it.
“We are the party that will be listening to them.”