How do you like them discoveries??? Prudie on slate.com has had quite a few questions about steps lately.
Typically, she is pretty disapproving of SMs.. but this one is about something else.

Q. Thanksgiving Big Reveal: My father had an affair on the side many years ago, and I found out about it. The affair had resulted in the birth of my half sister, Annie. Dad has successfully kept Annie a secret for 25 years. She's now a young married mother, and we are somewhat close friends. While my mother knows of Annie, she does not know she's my sister. My parents divorced when Annie was 5 for unrelated reasons, but my parents still get along quite well. I host Thanksgiving each year. Mom and Dad are coming. Annie just told me that she would like to be invited to Thanksgiving. While I have explained the situation to her, Annie says she's tired of being kept a secret. I'm worried of not only causing discomfort for my dad, but humiliation for my mom if Annie decides to let the cat out of the bag. I have tried to tell Annie that we could have a get-together some other time, but she insists. What should I do?

A: You surely want to avoid a pumpkin-pie-throwing contest on Thanksgiving. There’s something so despicable about trying to keep the existence of certain people a secret for the sake of other people. How hurtful this situation is to Annie and how humiliating it is for your mother. You’ve got little time to address this so that Thanksgiving is not a debacle. Tell your mother the truth, and set up a meeting for you, her, and Annie. Yes, it’s going to be a shock, but she’s been divorced from your father a long time and it’s well past the point that she should know you have a half sister.