Family Grieves Tragic Loss of Third Child to Genetic Condition Isabella Mading, 10, died on Feb. 6, years after her brother and sister.

 -- A Pennsylvania family is mourning the loss of their daughter after she died of a genetic syndrome that also took the lives of her brother and sister.

Erin Mading of Pittsburgh told ABC News that her daughter Isabella, 10, died on Feb. 6 from a brain cancer that was caused by the genetic condition Lynch Syndrome 3, also known as Constitutional Mismatch Repair Deficiency, or CMMRD.

"Everywhere I go, there's reminders of her and it's hard," Mading said. "I'm never going to go to the little girls department [again] to buy something for her. It's such a feeling, like it's a dream. It just doesn't seem like she's gone. I find comfort, especially with Isabella, that [the three of them are together]."

Isabella's brother, Cody, 17, died Aug. 31, 2010 -- just three years prior to the death of Mading's other daughter, Averi, also 17.

Averi died on Dec. 24, 2013. All three children died from brain cancer caused by CMMRD.

"Cody was stubborn and he had the biggest heart," Mading said of her oldest child. "Averi, she was the sweetest child. She was stubborn too, but not as stubborn as Cody. She was such a sweetheart and she had a heart of gold too. She fought so hard. Of all my kids, Averi suffered the most. She fought with everything she had."

"Isabella, she wanted to be a nurse when she grew up," Mading continued. "She had the best last year of her life. She was a social butterfly. From the moment she woke up, to the moment she went to bed, she was always on the go. She was silly and fun and so kindhearted. When I look back at all three of my kids...they lived life to the fullest."

Dr. Shawn McCandless, director of the Center for Human Genetics at University Hospitals Case Medical Center in Cleveland, Ohio, told ABC News that while the Madings' situation is “very rare," it happens when both parents carrying the same exact gene have kids, which heightens the risk of cancer.

“The cause of Lynch Syndrome is to have a mutation in one of the genes that’s important for repairing mismatched DNA,” McCandless said. “The issue for this family is that the parents clearly are both carriers. They have defects of the same gene so for them, every time they have a baby, there’s a one-in-four chance that the baby would be affected and that’s the same if you have parents with cystic fibrosis or sickle cell anemia."

“To have three of their five children affected, it’s actually not that surprising once you know the probability is one-in-four pregnancies,” he continued. “They seem like a really amazing family and it breaks your heart to read their story. It’s a very sad and unusual situation.”

Mading has two other children, Kylie, 20, and Olivia, 14.

After her son Cody was diagnosed with CMMRD, all of the Mading children were tested for the cancer-causing gene mutation.

While Olivia's results came back negative, Kylie has opted out of testing -- a decision that Mading said she supports.

"[She] still does not [want to be tested]," Mading said. "She doesn't want to know. I think she was afraid. She was 14 when Cody got sick. Kylie is 20 and I respect her decision. The funny thing is, we don't have anyone in our family who had cancer. People say, 'Why would you have five children if you knew about this?' but we didn't know until Cody was 16 and we already had all five of our kids. I never thought, 'Why me? Why them?' I think God has to have a bigger purpose. There has to be a bigger purpose of all of this. I want my children's lives to make a difference and I want to do everything I can...to help find a cure. That's what I am dedicating my life to now."

Mading said she plans on helping to develop more grief counseling for families and increase funding for CMMRD.

Since there was no awareness ribbon for the disorder, she and her family created one with a cheetah print design.