Haim claim they sacked their agent after finding out they were paid 10 times less than a male artist performing at the same festival.
The band say they initially "didn't think twice" about their pay, because they hoped playing the festival would help them get noticed.
"We later found out that someone was getting paid 10 times more than us," singer Danielle Haim told Grazia magazine.
The sisters didn't name the festival.
Alana Haim said the band - sisters Este, Danielle and Alana - had to defend themselves in the music industry.
"It's scary out there and [messed] up not even to be paid half the same amount. But to be paid a tenth of that amount of money? It was insane," she said.
Danielle added: "We had been told that our fee was very low because you played at the festival in the hope you'd get played on the radio.
"We didn't think twice about it, but we later found out that someone was getting paid 10 times more than us.
"And because of that we fired our agent."
The Los Angeles trio previously told Radio 1 Newsbeat they are encouraging more women to work in the music industry, especially in technical roles.
"We do see women working in the studios but we want to see more. It's a quantity issue," said Danielle.
"There's so many amazing women that work in music and I think it's time more girls looked into being engineers and studio technicians."
"We'll go to alternative radio stations in the States and there won't even be a girls' restroom," Este said at the Brits.
"That's a clear indication that something needs to change."
The Want You Back singers are currently in the UK as part of their world tour.