Even the twin's way of speech doesn't strike me as a twenty plus years old. Besides the narration and the twins' words from their own mouth, there's nothing about them that suggest they were isekai'd or twenty plus years of age. It wouldn't have been less frustrating to read if they got rid of the isekai part and just went ahead with actual children like Monster Duchess.
And I can't believe this story is adopting romcom's style of misunderstanding. Is the author that inept?