Why it is 'ist ihnen klar' and not 'bist du klar'?
"Bist du klar" is not correct german. You could say: "Ist dir klar, dass du zwei verschiedene Schuhe trägst?" The sentence with "Sie" is more formal.
so the sentence (more or less) is 'is it clear to you...' thats the reason for 'ist ihnen'? rather than 'do you realize'.
I understand the English sentence, but i just want to understand better the german construction.
ist ihnen klar ..... is 'do you realize' ????
See it with the second person... "ist dir klar" it would literally translate as "is it clear for you"... I think that "Do you realise" is pretty near to that meaning, even if not literally.
I hope this helps.