Worst Thing Sibling Can Steal

Listen up, you absolute melon. Your brain is softer than a bowl of porridge, so let me break it down for you. You see, the "joke" is that the punchline takes something really personal like virginity, which is generally thought of as a private and sensitive thing to lose. This makes it unexpected and shocking. Also, the fact that it is your sibling who takes it, well, it's a taboo. Do you get it now? Probably not.