I did an analysis with my family Except it was more like anal, y sis.
This seems like a strange joke to make about your own family. What made you want to tell it?
I've always been told to speak the truth so I will. It's fine to fuck your sister or any family member.
Joking about something as serious as incest can cause a lot of harm to people who actually suffered from it. Incest can traumatize people and cause large issues for children who are born because of it. You should read up on the topic to see the ramifications of incest so you can better understand why it's not good to joke about it.