Ich will alles haben!

Herr und/oder Frau Günther ==> Sie ==> your [formal]
Katja oder Georg ==> du ==> your [informal]; Katja UND Georg ==> ihr ==> "you all's" [informal plural]
Miguel Cabrera ==> er ==> his; Lady Gaga ==> sie ==> her; die One Direction Kinder ==> sie ==> their

You want everyone's stuff! You are talking to Herr & Frau Günther [to whom you say "Sie"], and to their kids Katja and Georg [to whom you say "du"]. Use possessive adjectives to indicate that you want the stuff of the people cued.
Beispiele: Katja Günther; das Haar ==> Ich will dein Haar! Herr Günther; der VW ==> Ich will Ihren VW!
Elvis; die Gitarre ==> Ich will seine Gitarre [you're talking to the Günthers about Elvis, so you say you want his guitar]