I'm years late to the party, but I feel like I need to add my two cents (I haven't read the following chapters yet.)
Yeah, it seems he is the only one having 'common sense', but let's not forget multiple things:
-He often calls out on his job, it's a miracle he still has one.
-He is a very bad guardian, why isn't his sister either at school or work? Apparently working is legal, hence the dark god job offer.
-He is extremely dense, come on, his childhood friend has been hitting on him for years apparently and he still hasn't picked on her. If he wasn't interested, he could acknowledge her advances and refuse. No answer is not an answer.
-And at least, even if he is dense to her advances, what is an excuse to accept her job offer with better condition and work than his current one? She has a booming business (she own a multiple stored house store in the town centre, she must be freaking rich) and he is afraid she just want to boss him around? Because that's not what his current boss is doing?
And my last point: maybe his perception is just really really bad?
Yeah, we see the dark god and the sword as demonic bad things. But we see them through his eyes. And what if his vision is twisted? After all, both his sister and his friend find it cute.
For him, it doesn't look like a sword, but maybe it does to everyone else.