I like this unique premise where two siblings are chosen to be both the demon lord and the divine hero. There's plenty of opportunity for both comedy and drama!
I'm also totally ready for some full on hate on the royal family for killing a/the goddess.
This kinda reminds me of that one manga that I didn't end up reading about some guy that's apparently simultaneously the demon lord and hero or something.
It seems a little bit backwards that the sword being pulled causes the demonlord to appear rather than the other way around. Maybe that will be explained later.