"Cheer up," a therapist might tell a couple when things look bleak.

But for some, cheerfulness and hope infuse everything they do. Such a man is Robby Wiemer of Havertown.

Wiemer, 35, and his wife, Carrie, 32, have four daughters, three of whom are disabled and whose lives have hung in the balance more than once.

"It takes faith in God and a positive attitude," Wiemer says. "When you see people in these life-altering situations, most people become depressed. We kept a positive attitude even in the worst of times."

Wiemer, a busy Havertown podiatrist, tells anyone who will listen about his "unique family," how identical twins Callie and Cassie, now 10, were born with Down syndrome and developed leukemia at age 2, within days of each other.

Then, daughter Kendyll, 7, was born with cerebral palsy. She uses a wheelchair, can't speak, and eats through a feeding tube. A fourth daughter, Ryleigh, 4, is the only one not disabled.

The chance that one family should be dealt such a daunting set of challenges is remote; that the same family should be happy despite the three girls' disabilities is even more remarkable, Wiemer believes.

He has used that theme to craft the outline of a book called Breaking the Odds: The Wiemer Family Story. Shortly after he mailed off the proposal, two book publishers offered contracts, and several others were interested, Wiemer says.

Philadelphia lawyer Eric Rathburn, who is handling the negotiations, says that as a first-time author, Wiemer is proceeding slowly.

"He's in the process of weighing the proposals and expressions of interest, and hopes to be able to make a decision sometime soon," Rathburn says.

Nothing could have prepared the Wiemers for the birth of their twin girls at Bryn Mawr Hospital in June 1996. There had been problems with the pregnancy, but nothing considered very serious.

"I will never forget the way in which he told us of our daughters' conditions," Wiemer writes of one doctor. "He told us they were breathing on their own, but he was concerned that they both had Down syndrome."

Down syndrome is a condition in which an extra chromosome causes developmental delays in a child. It is present in one in every 800 births, medical experts say.

Leukemia is much more common in children with Down syndrome, says Mary E. Norton, a member of the department of obstetrics, gynecology and reproductive sciences at the University of California, San Francisco.

"I don't exactly know how that changes with identical twins; it is probably more likely that the other will develop leukemia when one of the twins does. However, it seems extremely unlikely that they would develop it within days of each other," Norton writes in an e-mail.

The new parents struggled with special feedings for the infants, who couldn't suck like other babies. Life smoothed out. Then in January 1999, Wiemer noticed a rash on Callie that doctors thought looked ominous.

Callie was rushed to Children's Hospital of Philadelphia, where she was diagnosed with leukemia.

"I can remember hugging Callie as hard as I could, and I told her that everything was going to be fine," Wiemer writes. Eleven days later, the young parents learned that Cassie was sick.

Intensive chemotherapy followed, making both girls ill, and the young parents took turns working or sleeping in the twins' hospital room in Florida.

About the same time, Carrie Wiemer became pregnant with the couple's third child. Ultrasounds revealed that the fetus' head was growing too slowly; the girl was born with brain damage from a virus she had contracted in the uterus.

Robby Wiemer, who was doing a medical residency in Florida, put it on hold. The family returned to Philadelphia to seek expert treatment for the twins.

"I made a decision early on," Wiemer writes. "These kids are not going to die. We're going to get through this."

Faith and sheer grit paid off. The twins beat leukemia, although the recovery took years. They now study in a mainstreamed classroom, and swim in the Special Olympics.

Kendyll has developed steadily, with the help of a special school in Philadelphia. A cochlear implant enables her to hear. She smiles and appears happy, her father says.

Ryleigh has perhaps the biggest challenge; getting attention from parents whose care is often needed by the other three.

"We are happy with our lives," Robby Wiemer writes. "Many days are still challenging, but we try to expose our kids to almost everything. There are no limitations, and great expectations."