But doctors then made a surprising discovery. Caitlin said her consultant told her that after “triple checking”, they realised that “every single marker that we’ve looked for is exactly the same”.
They concluded the sisters were identical twins.
“I always wished I was an identical twin – I liked being a twin, but being an identical twin is a bit cooler,” Grace said.
Caitlin and Grace were in different amniotic sacs in the womb, so their parents believed they were fraternal twins. However, identical twins can, and often do, develop in separate amniotic sacs.
While the news came as a surprise, it also complicated Caitlin’s treatment. Doctors advised Grace could not donate stem cells because a successful transplant relies on the body recognising donor cells as foreign so they attack any remaining cancer cells.
Since Caitlin and Grace have near-identical DNA, Caitlin said “the whole concept of the transplant wouldn’t work”.
The transplant, which had been planned for August, was cancelled while doctors searched for another donor. During this time, Caitlin learned the leukaemia had spread to her skin and joined a clinical trial in Manchester to treat it.
Although the trial was successful, the cancer returned shortly before a second transplant date in December, meaning she required additional full-body radiotherapy.
“The radiation was probably the worst,” Caitlin said.
“I didn’t feel anything from chemo initially, but the first day of radiation, oh my God, I thought I was dying. It was crazy.
“I had, like, all the side effects on one day, but then by the second day, I got used to it, so I felt alright.”
In December, she underwent a successful stem cell transplant from an anonymous donor.
Both sisters are now encouraging others to register as stem cell donors.
“It takes maybe six hours maximum (to donate), and then for someone else, it changes their life,” Grace said.
“So I don’t see why you wouldn’t do it.”