I'm not sure which storyline you mean, but if it's from the extreme future, then it's not even anything like standard English. It's a mutated version, but still enough for a native English speaker to understand. If you think of a physical yawn and add it to the word "yarn" (figuratively: story), you can understand it as "I told you all about."

Yawned you could mean 'bored you sh*tless talking about him ... it is the scriptwriter's licence, means not much, is not common usage

@Neil Thank you for your answer as well!

I have no idea what Cloud Atlas is about, or why someone would have said "this is my big bro I yawned you about" in the context of the movie. However, sometimes people play with words. I can imagine a situation in which a person is talking about someone or something which he considers extremely boring or insignificant, and just for fun uses the phrase "yawned about" instead of "warned about" in order to indicate the boringness or insignificance of the person or thing he's speaking about.

haha, it's not "warned". the person who said this is like she's introducing his brother to the guest. so I guess it means "tell". i just don't understand why they use such a word? do westerners (movie makers, writers or any other people of such kind) make up words that doesn't exist to imply a different language habit in a different world of different places/times?