Stacey Andrew was near the front of the stage when she was struck by the lit projectile, which ignited her clothing and burned her skin underneath.
She was rushed from the venue, the FlyDSA Arena in Sheffield, South Yorks, with severe burns to her chest and arms as well as damage to her hair.
The identity of the individual who threw the flare is not known, but they are thought to have been located further back in the crowd.
A number of other concertgoers have taken to social media to say there were lots of flares set alight in the crowd throughout the concert on Monday.
Stacey's brother Leon has appealed for help to catch those responsible, who he claimed "could have taken my sister away from me and my family".
In a Facebook post shared with Gallagher, Leon wrote: "Anyone who has information on the scumbag who decided to light a flare and throw it into the crowd and set my sister on fire I want
his name or information on him because he needs to pay for what he has done."
Leon said he was at home when he got a call to say his sister, who's from Boston, Lincs, was in hospital.
"She will be scarred for the rest of her life," he said.
"All I've been trying to do is post everywhere I can on social media to find out who did it.
"Those flares shouldn't of been in there."
He added: "I want justice for the pain they have caused my sister because she will now have scars for the rest of her life."
The incident is reported to have taken place at the start of the concert while support act DMA were playing.
Another Facebook user shared a story from her children's experience at the same gig.
Sharon Gibb Vanes posted: "My 23-year-old son took my 12-year-old daughter as she loves Liam.
"We bought her the tickets for her birthday and she was so happy last night we purchased the tickets seated as we knew under 14s couldn't be in standing.
"I never thought for one minute she wouldn't be safe but how wrong I was.
"A fight broke out in the seats behind and one of the men came flying over seats kicked her in back."
Gallagher is currently at the start of a UK-wide tour.