The legendary novelist, who died this past weekend at the age of 77, spoke to People magazine about her cancer diagnosis.
"It would have really affected [Joan]," Collins said in the interview. "I just felt like she didn't need it in her life. She's very positive and very social, but I'm not sure how strong she is, so I didn't want to burden her with it."
Collins finally broke the news to her sister during a recent trip to London.
"[Joan] was very shocked...but she was great. We were emotional," Collins noted.
Collins was diagnosed with breast cancer in 2009. She only shared her diagnosis with her daughters.
"I didn't want people's sympathy. I think sympathy can weaken you. I don't live my life that way," Collins added.
Collins told People she had found a lump in her breast two years before she saw a doctor.
"I thought, 'I'm not dealing with this' because in my mind I decided it was benign," she added. "That was my choice and maybe it was a foolish one, but it was my choice."