A family member told DailyMail.com that Stefanie and Stacey remain hospitalized in Nassau.
The company that ran the trip, 4C's Adventures, declined to comment on the explosion to DailyMail.com because they are under investigation.
Stefanie is currently too unstable to be flown back to the United States.
"She's in really critical care right now and in really serious shape," the relative said. "I know she has really bad internal injuries."
She said Stacey shattered one of her legs in the explosion and suffered multiple broken bones and internal injuries.
Brooke and Bender managed to both escape with only scratches and bruises.
Among the others injured in the blast was teen Haiden Jones, his brother Kyle Ackerman, his mother Sheila Jones, and her boyfriend John Inman, Haiden's father said on Facebook.
"Prayers needed my son Haiden was on vacation in the Bahamas with friends and family when their charter boat exploded and caught fire! My son Haiden his brother Kyle his Mother Sheila, boyfriend John, girlfriend Brooke and Paul are all ok just cuts and bruises.
"Many prayers needed for Stacey Bender (Schaffer) Stephanie Schaffer last update Stacey and her daughter Stephanie still in the ICU," CJ Jones said.
The relative said Stefanie is a "very outgoing" dancer who recently graduated college.
"The scary thing is Stefanie doesn't know that she's lost her legs, and that's what really upsets me," she said.
"She's going to wake up from this coma and realise 'I don't have any legs'. It's breaking my heart."
A GoFundMe campaign has since been launched by the family to raise money for Stefanie and Stacey's medical expenses.
The relative said she is hoping Stefanie will soon be stable enough to be flown back to Florida for further medical treatment.
Stefanie's father and brother are both en route to the Bahamas to be with her.
"Hang in there Stef I'm on my way," George John Schaffer, Stefanie's brother, wrote on his Facebook Sunday night.
Four of the injured Americans on the boat were immediately airlifted to a hospital in St Petersburg, Florida with the help of the US Coastguard after the explosion.
It is not clear if the American woman was killed in the blast or if she died later from her injuries. Her name has not been released.
The explosion sent flames and plumes of black smoke soaring into the air, as seen in a photograph taken by Instagram user Chris Topperwien.
Several people on a nearby boat jumped into the water to help and dragged at least one person to safety.
Exumas and Ragged Island MP Chester Cooper confirmed that a woman had died and that two people, both Americans, were in an "extremely critical condition" in the Nassau hospital.
It now appears that those two Americans are Stefanie and her mother.
"This is a sad day for Exuma and a sad day for our American friends," Cooper said in a statement.
"We are saddened and our hearts our hurting, but we will get through this together. Your prayers, as always, are needed and appreciated."
The cause of the explosion is under investigation by the Exuma branch of the Royal Bahamas Police Force.
