Ok so either the death of The Queen or the death of Prince Charles will see William become The Prince Of Wales and Katherine almost with 100% certainty The Princess Of Wales
George will not then become Duke Of Cambridge, he will simply remain Prince George but more likely use Wales as his "school surname" rather than Cambridge as he does currently.
Neither Prince William nor Prince Harry became a Duke til they married the same will be true of George and Louis.
What Dukedoms they end up with depends on many factors
If William is King when George reaches 21 or so, George will become Prince Of Wales at that point as Charles did.
The tradition is for the oldest son/heir to be Prince Of Wales, if Charlotte had been first, she would have been the first to be Princess Of Wales in her own right because of changes to the succession.
The second son is always Duke Of York, if Andrew/Charles are dead when Louis reaches 21, he will become Duke Of York with Harry retaining Sussex, at this stage the Kings brother.
Charlotte will become The Princess Royal