You're not wrong. You're just missing the problem. And it's cool for you to say what you think would do in that situation, but respectfully, you don't know. Like if your stepdaughter came to you and told you what she was thinking, I believe you would do what you're saying. However, if you've gone 10-20+ years caring for a kid, always questioning whether or not that kid truly loves and appreciates you, you're probably going to feel a way when that kid tosses you to the side on a whim.
To be clear, I don't think the issue is that she wanted her biological father there. The issue is how she handled it all. She did it in a way that would inevitably make her stepfather feel jilted. After her fiance convinced her (I don't know why that nigga added his two cents in at all) to include her real father, she should have gone and had a one on one conversation with her stepfather. That alone might have been enough to avoid the mess, and it was the obvious thing to do. I believe she didn't do that because she knew there was a possibility that he might refuse to pay for the wedding if he knew ahead of time.