Secret Family Member

My great-grandpa has another daughter nobody knows about, except me. I talked with him one day after he had his drink, and he looked at me and said, “Listen, before I die, someone has to know this, I have a daughter that nobody knows about. I’ve had nothing to do with her or anything. Keep this to yourself until I die.” I nodded and told him I would. He is still alive, and we don’t talk about it.