@Speederzdk
Hmmm... what would I do? Well, let me preface by stating that I am aware of just how stupid I am, that I am also susceptible to mistakes and bouts of idiocy. What I say might well be as dumb as what MC has done, but, I would like to believe that I have given the matter some thought, instead of blurting out a knee-jerk reaction. In other words, these next words might be dumb, but there are reasons to them, which is better than none.
Anyway, in chapter 5, I think, it's stated that the sister is 14. Since MC is 2 years older, he is 16. At the time of his first life, he lived until he was 17. So in this new life he is 16 years old, but, mentally, he should be 33.
Now, 33 might sound too young, but in reality, if you got married on your 19th birthday and had a kid before your next birthday, by the time you're 33, that kid would have been 14 years old.
This is to say, by 33, you really should be a responsible adult, with thinking abilities far beyond just reacting to whatever is happening to you.
Yes, MC did get his growth stunted somewhat, considering he died and reincarnated and all, but by 16 years old in his second life, he really should have been able to catch up. If not 33, at least he should be 25 years old mentally (yes, I pulled that figure out of my ass, but 25 seems a good approximation).
So what could we expect a 25 year old (or 33 year old) to do? Well, for starters, think a hell of a lot more. Don't have time? Then stall for time.
By the way, leaving in the dark of night, saying "I won't come back until either she gets married, or i get married" is not stalling for time. He might as well have announced that his sister is an untouchable bitch, not fit for anyone to marry.
I mean, how else are people supposed to interpret that kind of abondonment? Imagine suddenly being assigned to the untouchable caste. What kind of things would people say about his sister? That's what really pisses me off the most, actually. He has the mental capacity to think of a solution - any solution - so that his sister wouldn't get hurt, but he didn't. What a piece of shit.
What would I do, aside from thinking? Well, try thinking of other things. Why does this world of ours forbid incest? Religious beliefs, as well as that thing with overlapping genes:
<See https://en.wikipedia.org/wiki/Charles_II_of_Spain>
Charles II (Spanish: Carlos; 6 November 1661 – 1 November 1700), also known as El Hechizado or the Bewitched, was the last Habsburg ruler of the Spanish Empire. He is now best remembered for his physical disabilities, believed to be the result of inbreeding ...
<See https://www.nationalgeographic.com/magazine/2010/09/tut-dna-dobbs/>
The physical problems faced by Charles and the pharaoh Tutankhamun, the son of siblings, point to one possible explanation for the near-universal incest taboo: Overlapping genes can backfire.
By the way, both articles are quite good reading. I recommend them, if you haven't already.
So, if the problem is religious, then check, does the problem exist in this new world?
Same thing with physical deformations due to incest - does it exist here?
If the answer is no to both, then figure out how to feel about marrying the girl?
Steps by steps. Either do what I just described, or whatever. There's more than way to scale a fish. MC could well choose not to marry his sister. BUT. Just don't fucking run off and let your sister get treated like a pariah.
