the MC was not his favorite child and if you carefully read the story when he was enrolling in the army he said that his father saw him as the heir to the company and that's why told him to go in the army to learn discipline, respect, and other things. But his sister was enjoying life fully working with their father and all that.
About the so-called joke, yeah, I saw when talking with the guys from HR that he said to him it's was a joke that went too long or something like that... but I don't recall any hint of joke when he told her to call him like that, instead if I remember it right, he even took a serious tone and held her accountable when she said "Whatever you say" and he said something like "Whatever you say, Who?"
Do you really think that his sister was his father's favorite? Did you even consider things from other people's perpective?
She was working her ass off for the company, she did everything for her father and even going as far as bringing him his morning coffee. And after putting all that effort and hard work she never received any recognition. Even with all of her work experience her father didnt even consider her as possible CEO. She was trying her whole life to get some recognition but she received none. Everything was given to his son.
If you think only from the son's perspective then it might seem like "right thing to do" for him to inherit everything. He is male, blah blah blah. But from the perspective of his siblings it's terribly unfair.
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I don't recall any hint of joke when he told her to call him like that"
What you have mentioned is one of the common problem of the sarcasm. It is very easy to miss the sarcasm in written text, because sarcasm is usually indicated by non-verbal signs of comunication. But unfortunately written text lacks non-verbal markers. That's why some people notice the sarcasm in text, some dont.
To me it was pretty obvious he was sarcastic while he said "
Whatever you say, who?". If he was 100% serious he would never say something ridiculous like "
my lord". He is no nobility, it isnt right for him to demand such things (higher social circles take their titles seriously). That's why I am certain it was definitely a joke. He also said it so himself to coworkers.