You know, I stopped at this chapter yesterday then went to sleep. Only to wake up thinking that our angel might actually be a devil and our devil might be a nephilim.
Okay, so, it is said that only devils have an ability (which summoning chains clearly is) and her sister who should know about it hates her for being a devil. Maybe their parents took pity on the devil who looked just like their daughter and adopted her.
Likewise, he does not have an ability but has wings which is something only angels are said to have, he's also physically strong as angels are. He can transform to a "devil form" tho, since we've seen devils doing this I'm guessing he is half of each.
There is also the thing with angels using dolls tho, so maybe she's a nephilim too, but since we don't know how dolls work maybe it's just technology anyone could use.
I just hope that if they both nephilims we don't have a Major Shoujo Site kind of situation.....
But, like I said, I think she's a pure devil raised (mostly imprisoned in her room) in heaven and he is a nephilim left in hell.