Timmy is Danny. Timmy made one final wish to be a super hero before Cosmo and Wanda had to leave him. This is why he has no memory of them in Danny phantom
timmy died but cosmo and wanda tried to save him it only kinda worked. He woke up in a slightly darker twisted world.....wait how the f did he get an older sister then did they adopt vicky?
Vicky, but nice. In that alternate universe, where Timmy made his last wish to be a superhero, Cosmo and Wanda wanted to give him a new parental figure, since his parents are really not that smart. So they took the evil butt-bug out of Vicky (bonus points if you get the reference) and turned her into Jazz.
Yep. Nice.