Substitution of "have".

"I have letters to write" means I have the TASK of writing...".

"<For instance, you can say "I have a sister.", but not "I own a sister." >

But you can say I possess. "

Saying "I possess a sister" would sound rather odd.

<<Saying "I possess a sister" would sound rather odd. >

uNLESS YOu WeRE A DEMoN. :-/