His “sister” is actually a demon, and that’s why Ducal Prince Nestro hates her so much.
It could be that House Nestro never had a daughter to begin with or that something happened to the ogRonella and the demon took her place.

Either way, Nella being a demon makes SO MUCH SENSE.

The reason of his brother’s hatred for her and the foul smell of demon he senses? Ronella being a demon.

Remember when she ran to the forest alone despite the dangers of a demon there? If she was human, there were too many variables and doing that would be too much of a risk just for a fish. However, if she were a demon, it’s totally reasonable because a demon has no reason to attack other demons.

Why is she wary of the FL? Because she’s a demon and our beautiful protagonist is the Saintess.

Why she feels uncomfortable at the temple? Because she’s a demon.

Why she wanted to see the injured Saintess after the attack at the temple? Perhaps Ronella wanted to see if she had an opportunity to be in contact with the weakened Saintess and take her down.