Keeping a secret from my sisters help I need help

Today my mom called to and had asked if I can go to her doctors appointment. I had asked if everything was okay. She started to cry and told me that the doctor told her she got breast cancer. Now we went to make sure it was what he told her over the phone. So my mom had asked how to tell my sisters and she said she wanted to speak to them on the phone today but I explain it's better if you to sit them down face to face. Now I told her we can have dinner this Saturday and she can speak to all of us, and what is the text step in her process to recovery and what stage she is in. Now I called my sister to let her know about the dinner this weekend and that their spouses had to be their as well. She asked if everything was okay. I said it's not my place to say but everything is okay as far as I know. But she keep asking and she knew i was keeping something from her. I wanted to tell her but i couldnt 1 i didnt want my mom to get mad. 2 i didnt want to tell her over the phone. Now my problem is how can I keep this to myself I should say something but out of respect to my mom I decided not to say why. Am I wrong for not saying? I know I would be mad if my sister knew and I didn't but is it my place to say? Advice need please.