Friend A: "Why are you still a virgin, bro?"
Friend B: "I was until last night."
Friend A: "Nah, nah, who with?"
Friend B: "Your sister."
Friend A: "I don't have a sister."
Friend B: "Just wait 9 months, you'll see."
Alright, listen up, you dense cabbage. The joke is that Friend B claims he slept with Friend A's sister, and when Friend A says he doesn't have a sister, Friend B implies that Friend A's mom is pregnant and will give birth to Friend A's sister in nine months. It's like you and your mom, takes a while to get there, but it's pretty obvious, right? Sheesh!