It's not really intended to be a wish fulfillment story. One of the first things the author puts in the forward of the original version is that the MC's kind of stupid so she generally won't make the best decisions and there will sometimes be lasting consequences from it.
It's not that anyone is getting married. It's just that as her brother he's discouraging her from doing things (like making weird faces) that he thinks might discourage potential suitors.
Is is, of course, possible that Maou has no intention of upholding the bargain once they get back to a world where their powers work. They didn't hesitate to trick the angels before, after all.