What I understood was one, the part where ageha says his sister likes him is a reference to the fact that instead of going after guys she can, she's fantasizing about guys she can't. Basically, her last romance went sour and embittered her until now. (i understood that page with the title as ageha's thoughts, not amamiya)
As for the part about oboro being killed...um, what? Going strictly by what he said, all we know is he bought the card (probably for some enormous amount of money). It's unused, and because of his intuition that it's dangerous, likely hints that it's a real one. Remember back to the beginning...there are many fakes floating around. Although the last line of ch12 didn't use the word drifter in it (i added that), it really is a big giveaway because it literally said something like "does this man share their fate?".