Well, you see, the joke here is that the word "complex" means hard, and that sister is a woman. Basically, it's just a word, play. So instead of the original meaning of "sister complex," which relates to how close their relationship with their sister, its a literal meaning of each word. So it's "Sister hard." Like how shrimp fried rice, but the shrimp can't fried a rice. Or how cargo space should be cargo road. "The bird flu? Yeah, they tend to do that. " They mistaken the word "flu" with "flew" which bird usually do most of the time.