User blog comment:B214/Volume 17 spoilers./@comment-187.209.0.247-20140306224029/@comment-76.18.188.198-20140312040549

The first part of your question about Grayfia and her brother. If you reread the Euclid spoiler above, he is obsessed with Grayfia. I would believe even a devil woman raised in the the Underworlds culture would call him "Creepy",  eventually.

The second question I would like to suggest reading one of the few short stories I actually enjoyed. It was "Lets Go to Onsen", at Code Zero's Blog. To put it simply Grayfia likes him, too.( I would not read anything else into the story, because the short stories normally don't really help the plot, anyway. An  I doubt she is going to move in or anything....etc, anytime soon, because of the impropriety of it. )