I am having doubts getting married because my fiance chooses his nephew over me: Advice?

So I’m getting married in a month and having a lot of doubts. So my fiancé has raised his nephew since he was about 2, and he also has a son with his nephew’s mother. He told me the reason why he was close to his nephew is because his brother died, and he has nothing left to remind him of his brother. I thought that his brother died when his nephew was a baby, however not the case; his brother died about four years ago. So he didn’t want to be alone, so he hooked up with his brother’s ex, and they have a son together. My issue is I don’t feel like I should be placed behind a child that isn’t his. For example, I asked for my fiancé to spend the day with me on the anniversary of my mother’s death. Instead, he spent the whole day with his nephew cause it was his birthday. I get that he is family, and I have told him numerous times that his nephew needs to stop calling him daddy. I am just super stressed to the point that I want to call the wedding off cause I will always be in second place. Am I in the wrong?

Sounds to me you are for some reason jealous. That child lost his father and your fiance stepped up, you should be so proud of him for that

The nephew was 10 when his father died and this kid calls him daddy.