They all have a brother or sister with cystic fibrosis, a disease without a cure that clogs the lungs and digestive system and makes it hard to breathe.

They are calling on the Macarthur community to support the Advertiser's swimathon and launch night which raises funds for Cystic Fibrosis Australia.

Georgia Free said the hardest thing about her sister Julia having cystic fibrosis was that no matter how well Julia looked, she always knew she was very sick.

"It breaks my heart to see her sick and I really want to see her get better," the 15-year-old from Elderslie said. "Mostly you can't notice that she's sick and a lot of the time it just seems like she's a normal kid but she has to go to hospital regularly to get her treatment."

Ten-year-old Jemma Harvey, of Bradbury, said she wished her little brother Corey didn't have CF because she didn't like seeing him get tired and upset when getting his treatment at night. "Sometimes Corey doesn't get to do stuff and then I can't do it too," she said.

Jemma said she explained the disease to her friends by saying that his "lungs don't really work properly" and she had asked her class at Broughton Anglican College to swim for Corey at the Macarthur Swimathon for Cystic Fibrosis this weekend.

Jeremy Bradney of Ingleburn, 17, has three siblings with cystic fibrosis. He said he had learnt to cope with it but hoped residents would get behind the swimathon.

"It's just to help them out and to make them better."

Fifteen-year-old Hayden Booshand said he also hoped lots of people would raise money for the cause to help his little sister Jaylee.

"It'd be a good idea to come to it because it helps out a lot with the disease."

The swimathon's launch night will be held tomorrow at Camden Valley Inn at 6pm and the swimathon will be on this Saturday at the Wollondilly Community Leisure Centre in Picton.