Melissa Laurie, 28, was saved from the attack thanks to her twin Georgia who punched it in the face three times and dragged her out of the water, She is currently recovering in a hospital in Mexico.
Georgia is also in hospital, being treated for her hand injuries.
Now their older sister, Hana Laurie, 33, has launched a GoFundme page to raise funds for their family to travel to Mexico and her sisters’ medical care costs.
Providing an update for Melissa’s condition she said “the next 2-5 days will be key but luckily she’s cut from the same jib as a CrocodilePuncher.”
Zookeeper Melissa was swimming at night in Manialtepec Lagoon, ten miles from surfing resort Puerto Escondido when she was dragged underwater by the crocodile and thrown “around like a rag doll”.
Georgia fought off the beast with a flurry of punches and swam back to the tour boat on Sunday night while their friends called for help.
Hana also said that the tour guide who was locally known to orginise illegal tours, has vanished since the incident.
She writes: “My sisters were taking a break from their ambitions to scuba dive and work with animals when they took a trip with a guide recommended by their hostel near Puerto Escondido, in Mexico.
“It’s still unclear exactly where they were, but a crocodile attacked the group and viciously bit and shook Melissa.
“It has since transpired that this guide was known locally to be operating in an unlicensed and illegal way, but since is nowhere to be found.
“We are so lucky that Georgia was there, and had immediate reaction through her water rescue training to fight off the crocodile and drag Mel to safety.
“The details of this awful ordeal are still not fully known to us, as the time difference, language barrier and the fact they’re both admitted to hospital are making it very difficult to communicate.
“What is clear, is that Mel’s condition is “delicate” having suffered an open wrist fracture, organ lacerations/internal bleeding and water on her lungs.
“Doctors are very concerned that she’ll contract an infection (as they are too about Georgia’s wounds) and have put them both on intravenous antibiotics.
“Melissa has been placed into a medically induced coma, and remains in that condition while doctors attempt to drain fluid and enable her to breathe on her own.”
Speaking on BBC Radio Solent, Hana said her siblings booked the tour through their hostel before adding that the tour guide was not registered – and had taken them to an “unsafe” swimming spot which was not approved.
“He’s an unlicensed tour guide and according to someone on a Mexican Facebook page he’s been doing it for a while, doing illegal tours in unsafe, unregistered areas,” Hana said.
“He’s gone to where the crocodiles live and not the location where all legal certified sanctioned tours take place.
“He has been called out locally before, apparently, but that wasn’t for my sisters to know.
“They found out the information through their hospital.”
Meanwhile their mum Sue Laurie, 63, said it was Georgia and Melissa’s unbreakable bond that saved their lives.
Speaking to the Daily Mail, Mrs Laurie said: “They have always been there for each other.
“The girls can fight like cats and dogs, but there is no closer relationship than twins. They will do anything for each other.
“And what happened in Mexico shows just that. If Georgia had not have been there who knows what would have happened.”
Manialtepec Lagoon is famous for its bioluminescent waters but swimmers are warned that crocodiles up to ten feet long live there.
Georgia and Melissa had been travelling around the world together since March and they were due to arrive home in November.
Following the ordeal, their parents are considering flying out to Mexico to bring them home.
Hana has raised more than £28,000 so far. To donate to the fund, click here.
Huge crocodile spotted prowling in the SEA just metres from beach popular with families