Birthmothers I need your advice

My mother was 15 when she had me. We met when I was 17. I was a closed adoption. She had an 8 year old and a 12 year old girl with a different man. She never wanted her kids to know. We lost contact for many years and I called her again at 34. I was very ill and needed medical history. She said she still not going to tell her adult children. Months later they found out and not through me, but when they contacted me I confirmed it. I now have a wonderful relationship with my Bsisters. We are happy to know one another.....this is the problem. She won't talk to her daughter about it. Flat out refuses to discuss it. We feel this gloom hanging over our heads and I want to talk about it with her. I need your advice and opinions on how she could be feeling. She has three very confused daughters. I heard a Nmom say that when people asked her how many children she had she would say 3 but it would hurt because there were really four. I understand (kind of) because I lost my second pregancy early on. I want to know what to say to make her stop all these secrets. I know she must have suffered. I was too young to understand when I met her the first time...I was told she gave me away because she loved me and I though she would be happy, but it triggered all this pain and shame. I don't know what to do.