A young woman was diagnosed with a rare and aggressive form of breast cancer - just two weeks before her older sister received the same gut-wrenching news. Katie Cooper, 28, of Charlotte, North Carolina knew something was wrong when she felt a 'golf ball' in her chest in September 2024. Biopsy results soon confirmed her biggest fear - she had stage two triple-negative breast cancer (TNBC). Shocked and devastated by the life-changing news, Katie contacted her sister, Lyndsay, who lives in New York City. 'She found a lump and called me to let me know and said "will you please go get checked,"' Lyndsay, 32, told KHOU11. 'That day I went to the gym and I was in the shower and I was like "I'll do a self-check" and I felt a lump and I said "there's no way." The sisters were diagnosed with the same strain and stage of breast cancer. TNBC accounts for about 15 percent of all breast cancer cases and most commonly occurs in women under 40. TNBC 'differs from other types of invasive breast cancer in that it tends to grow and spread faster, has fewer treatment options, and tends to have a worse prognosis,' according to the American Cancer Society (ACS). Survival rates vary depending on the cancer's stage. For stage two and three, 60 percent of patients live past five years of diagnosis, the Cleveland Clinic reported. However, once stage four is reached, the chances of surviving after five years plummets to just 10 percent. 'It grows really fast and is really aggressive so I feel so lucky we both caught it super early,' Lyndsay told the outlet. 'And it's insane, the tumor is the same size, same staging, same course of treatment, so it's really, really bizarre.' The sisters maintain the importance of breast cancer awareness, as they believe their diligence is what helped them catch their illnesses in their early stages. They both carry the BRCA1 gene, a mutation the increases the risk of breast cancer. 'When I was 25, I did 23andMe mostly to figure out, because we have two moms and they used a sperm donor, we always knew there were probably half siblings out there,' Lyndsay told KHOU11. 'We found out we were 50% Jewish, Ashkenazi Jewish, which is a risk factor for the BRCA1 mutation. 'From there I was getting regular MRIs and getting checked out.' When Katie went to her health care provider in September, her appointment was with the same nurse practitioner she had seen just three months prior. 'This definitely was not there three months ago,' the nurse told Katie, emphasizing the importance of staying alert when it comes to her health, TODAY reported. Katie and Lyndsay have decided to document on social media their unique and taxing journeys as sisters battling cancer. From head-shaving to wig-buying and chemotherapy, the sisters have offered their more than 17,600 TikTok followers glimpses into their lives' most vulnerable moments - all while keeping smiles on their faces and even cracking a few jokes. 'With TikTok, its been wild to hear so many women messaging us: "I'm in my 20s, I'm in my 40s and had cancer,"' Lindsay told KHOU11. 'I don't want this to happen to anyone else, so if I can encourage people to get tested for BRCA1, even if you don't have a family history, it's more common than people think and just knowing and being able to go get preventative checks gives you such a peace of mind.