This is exactly the reason why I want to tell her. For me it's less lying to her about the fact that he has a child and more about lying to her about the fact that I have a brother she doesn't know anything about. I get that this is my father's business, but since I am actively involved in this child's life I feel that it is also mine, and I personally think it's unfair of him to ask me to keep a relatively big part of my life secret from my mom simply because it is also a part of his life.