Romi Loffler, 7, of Giffnock, East Renfrewshire, suffers from Rett Syndrome, the same condition from which Rosie McLoughlin, 14 – Colleen's sister – died at the weekend.
Mrs Rooney, wife of Manchester United striker Wayne, was very close to her sister and has spoken of her heartbreak.
Romi's mother, Gael, said Rosie's death had made the family more determined to raise funds to help find a cure for the condition.
Rett Syndrome causes children to lose their mobility, their speech and the ability to do anything for themselves. Few sufferers live beyond their 40s.
The disorder affects about 12,000 children in the UK, almost all of them girls.
Gael, 42, said: "It's heartbreaking. Our heart goes out to Rosie's family. We hugged Romi that little bit tighter when we heard."
Romi was diagnosed with Rett Syndrome at about 15 months. Her parents had noticed she was struggling to reach the milestones most toddlers do, such as walking and talking.
Gael said: "There was nothing in the first year that hinted at anything out of the ordinary. It was when she was 15 months that she started missing her milestones.
"She was very late walking. She does walk now and that's very unusual. Many children lose that ability.
"She can't take care of herself. She can't talk and she can't feed herself. She also suffers from reflux, which can make her unconscious."
Most heartbreaking for parents is that children lose the ability to communicate, in a similar way to 'locked-in' syndrome.
Gael said: "Sometimes Romi bursts into tears because she can't communicate what she is feeling.
"When you are told your child has Rett Syndrome, it feels like your world has ended, but Romi is doing remarkably well. She is always smiling. What she really loves in music. She loves the Beatles."
Romi communicates using Eye Gaze technology at the Isobel Mair School in Newton Mearns, which allows her to use a computer using eye movements.
Gael and 45-year-old husband Gunter, both lecturers at Glasgow Caledonian University, are raising funds for the technology for their home.
Romi is close to her brother Felix, 11, and sister Ella, 10, who are adept at understanding what she is trying to say.
However, there is hope on the horizon for families like the Lofflers. Research in Scotland in 2007 showed Rett Syndrome can be reversed in mice.
Gael said: "We are very hopeful something can be done that will help Romi.
"The news of Rosie's death makes parents like me more determined than ever to do everything we can to raise funds to keep the research going – until the day they find a cure."
Donations can be made to Rett Syndrome Research Trust UK at: www.reverserett.org.uk