Why is it called "Devil Sister" ?

 Greek Irene:

The show focuses around an age gap couple and their problems with being seven years apart, but the show is called "Devil Sister" which goes completely against the logline.

I realized by the end of episode 3 that the title most likely doesn't refer to the couple, but more so Inn. The literal sister. I made another thread around the airing of episode 1 called, "What's Inn's deal?" Which everyone there has made up such good points about Inn. 

As much as we all don't want Inn to be just the antagonist role, I'm feeling that this series is either going to full out make her the antagonist(as we're already getting a glimpse of in episode 3), OR they're going to focus more about her character overcoming her struggles. 

So further more I think the TV show title "Devil Sister" is about Inn.

They are actually referring to Irin and not Inn since Irin has the devil's tail and horns on the promotional poster. It is supposed to represent Irin's personality on how she is very hot-tempered towards other people including her sister but not Namcha since Namcha knows how soft she really is in the inside. Furthermore, many people think that Inn is soft as she is, so she is not to be feared unlike Irin.

 Greek Irene:

LMFAO I LITERALLY FORGOT IRIN WAS A JERK.

LOL! I AM DONE! There is never a moment I forget.

 Greek Irene:

I was too focused on Inn ?? 

I understand, unlike Irin, Inn has some many complexities within her character. Which is why for me, I would understand considering she does go about things the wrong way.