To know if you're a daughter in the manga, we'd need to look at how the author has developed the characters and the storylines. Maybe there are certain clues or indications that could tell us. But as it stands now, it's impossible to say for sure without more information.
One of the main themes is identity confusion. Since the character says 'I am my wife', it implies a blurring of the self - identity. Another theme could be the exploration of relationships from a different perspective. When a person is in the role of their wife, they see the relationship in a new light.
Since I don't know the exact content of 'I am my wife' light novel, it's hard to say for sure. But possible themes could be identity, self - discovery, and relationship. If the plot involves someone being in the role of their wife, then identity would be a big part as they are taking on a new identity. Self - discovery could come in as they learn about themselves in this new role. And relationship as it would surely affect their relationship with others in the story.
It's not that simple to tell. You could be the strongest based on your unique abilities and how they compare to others. But also, the context and plot developments can influence this. Maybe you are, maybe not!