I think the first thing you both need to do, though, is to adjust your attitudes in the situation. I know this sounds a bit accusatory, but hear me out. You describe Emma, who is a child, as a "spoiled brat." You don't just see her behavior as bratty, but her personality as bratty and somewhat irredeemable. Children can pick up on when they aren't liked, and-like it or not-when you married Diana, you also made a commitment to Emma that includes being there.
"Number one thing I say is get a stepparenting therapist or stepparenting coach, because I got that right when we got engaged, and it's been incredibly helpful for me and also just understanding my role as a stepparent."