I am about to start a long running campaign with a bunch of friends and because of the predicted length, several of my player wrote me several pages of backstory and I really want to incorporate them into the campaign, however due to lack of time (and to be honest, lack of expertise) I don't know if ill be able to write a full 1-20 campaign before the first session. So I am contemplating taking a pre-written adventure (like Tyranny of Dragons, Out of the Abyss or Princes of the Apocalypse), but I don't now how I could include brand new cities, tribes, locations, people and to be honest, a completely different world, into a pre-existing adventure.
[RPG] How to seamlessly incorporate character backstory into a pre-written campaign? D&D 5e
The GM takes on more work than any individual player in making the world and the game come to life. Obviously a GM without players has nothing, but the lynchpin of the game is still the GM. Having played in many games and GMed many more, I come into any game with the understanding that because the GM has to do so much work just to get the game going, that person should be given the benefit of the doubt where possible.
Your player's reaction to me seems a bit off, in that you were just getting back into GMing and this was the first session in a new campaign. To expect everyone's expectations to align perfectly, and the session to play out without corrections or adjustment is asking quite a lot.
For detailed background, I generally provide overview text and invite the players to ask questions as they come up. It's perfectly acceptable for a player to say, "I figure these thugs should be licking my boots because I'm a member of the guard. Is that right?" This approach requires that the GM be able to fill in details on the fly, but it also puts some of the responsibility on the players. Some players may bristle at this approach because they find it too meta, but the necessity of using it fades away as the campaign develops and the players become more immersed in the world.
This is fundamentally a question of playstyle
In differing styles players have jurisdiction over differing amounts and kinds of fictional material. In some styles, it would be completely inappropriate for you to determine any aspect of the PC's brother's character. In others, it would be completely inappropriate for the player to decide that his character even has a brother or any aspect of that character, should they exist.
Most of the time, there's some sort of balance between player and GM authority on most issues. As a matter of playstyle, groups might resort to the rules, out-of-game social hierarchies and methods of discrimination, shouting matches, open discussion and compromise, or any manner of other methods to determine the nature of the game world when there is a dispute of some kind.
Your asking this question makes it clear that your group hasn't yet formed a position within this question's scope for the purposes of your game. That's not a bad thing-- playstyle is often most meaningfully developed during play. That said, you have correctly identified this as an area of concern: your player has invested a lot in their PC's attachment to the brother, and if it goes 'wrong' people may well be upset. The solution here is to talk about what you guys want as a group regarding fictional authority over PC backstory relationships as well as PC backstory relations. You don't need to specifically voice your plans with the group regarding the brother turning out to be the BBEG or whatever you want to later be a reveal; discuss the stance of the group in a general sense, for example by asking "Is it okay for me to keep information about important characters secret from you in order to reveal it later" and "How much control over your brother should I have". Ultimately, figuring out how you want to deal with this will be a formative process for your group.
I'm going to offer what I think is a fairly simple option: work with your player(s).
They have clearly got some investment into your game, so I think you should talk to them and tell them what you have here. Tell them that you don't have a lot of time to come up with your own world and are planning on running an pre-written campaign in a pre-written world.
Then, you ask for them to help integrate the backstories they gave you more deeply into the world. I haven't been a GM in this situation, but have played in several long-running campaigns and have always worked with the GM on the backstory. The backstory has always ended up better, being more focused and connected to the world than if I had just written a few paragraphs and handed it off to the GM for planning.
So in short, leverage their interest and creativity to help you by involving them in the process more deeply, which in your case is introducing them to whatever pre-written world you are looking at so they can tie themselves into it, rather than leaving you the work of writing all of their backstory into the world yourself.
I believe at least Tyranny of Dragons has included material to help link characters into the story more deeply (e.g., a character might know some other important character in the story), so you might scan for that kind of information and use it to link into their backstories. For example, if one of the suggested bonds in the module is that the character is looking for a childhood friend, and if one of the characters talked about a town they were from, you might suggest it actually be that town.