OP, having read all the comments and your post I strongly advise you not to go the primary reason is that you cannot afford to do so. You would need to miss work and school to do so and I don't think it would be a wise decision considering how strapped you are for cash. (But kudos for doing work and school, I know how difficult it is. Keep it up. You can do this!!)

Another thing is that, it sounds like you are the after thought to your father's new family, which to me is a big flag. You mention your father makes a six figure salary, yet it all gets spend to your siblings and his new wife. You? Irregular child support and it sounds like just half-assed attempts to be a parent to you. They know you are in college, working and do not have a lot of income yet expect you to magically come up with $700 to fly to the other side of the country as well as get a dress? What about other costs like food and where you stay? Furthermore, if you had known about the situation months ago it would've been more under your control as you would've realized about this cost. But you didn't and it sounds like your father is throwing you under the bus.

I suggest send a card or a nicely written letter to your brother and simply mention that when you can afford it, you would like to take them out for a celebratory lunch the next time you visit them.

They may be related to you, but it doesn't mean you have to set yourself on fire just to keep them warm. If they refuse to understand or accept that you cannot afford to go to their wedding (a very reasonable excuse and you have every right), I would revisit exactly where you stand with them and vice versa.