Melissa and Georgia Laurie, both 28, from Berkshire, were swimming in the day time in a lagoon in Mexico, around 10 miles from Puerto Escondido, when Melissa was attacked by a crocodile.
Their sister, Hana Laurie, 33, said Georgia saved Melissa by punching the crocodile in the face when it attacked her sister three times.
Both of the twins were taken to hospital in Mexico, where Melissa has been placed in an induced coma and has since developed sepsis as a result of her injuries.
Georgia has been discharged from hospital.
Hana told the BBC on Wednesday: “I spoke to Georgia overnight, she’s doing much better, she’s really frightened and a little bit high on adrenaline and pain killers… but Melissa’s situation is still really serious, we’re not relaxing just yet.
“She’s still incubated, in a medically-induced coma and she’s now developed sepsis. She had a lot of water and grass in her lungs because crocodiles try to drown people.
“She’s got some bite wounds across her tummy and her legs, some of those have become infected as well. It managed to tear her bowel so [it’s] quite stressful.”
Hana previously said the pair had booked a tour with a guide through their hostel and the women found out later their tour guide was not registered and had taken them to a swimming spot which was not recommended.
“They asked if the water was safe and were told it was, and evidently that’s not the case,” she told BBC Breakfast.
“They’re not naive people, Georgia and Melissa are experienced with animals, Georgia’s a great diver, which is how we think she’s managed to save her life. It’s magic twin stuff meets her amazing training in water safety and just a real big dose of that famous Laurie girl attitude.”
A GoFundMe page set up by Hana to raise money to help cover medical costs for the twins has raised nearly £38,000.
Writing on the page, Hana said their father Sean is due to fly out to Mexico to see the girls in the coming days and the family has been put in touch with an English-speaking doctor, Dr Esteban Bolanos, who is helping give the family updates on Melissa’s condition.
Describing Melissa’s injuries, she said: “Mel is very bruised, has two black eyes and deep cuts to her face, torso and leg where the crocodile has bitten and scratched her.
“There’s currently no change in information for Melissa’s condition which remains ‘delicate’, but we think stable. We wait to see how she reacts and recovers over the coming days.”