Welcome!
To elaborate on what everyone is saying about the first few pages being rough in a book. This is a phenomenon across most of Japanese writing, not just books, but books, especially novels, biographies and other narrative works, are, in my experience, where the feeling is starkest. Japanese is incredibly context heavy, so once you have context, sentences can be as short as just the verb, but without context you have sentences with a dozen particles, and subclauses two lines long in order to make the sentence even make sense at all.
One of the first light novels I read, ゼロの使い魔、opens with a sentence that starts by talking about the sky behind a person, goes on to talk about the appearance of the person in front of the sky, the narrator being on the ground, the landscape surrounding them, and eventually gets to the point of “someone I don’t recognise is peering down at me and asking me a question.” As someone who had just gotten into Japanese fiction, I was very confused at first.
Jay Rubin puts this excellently in his ‘Making Sense of Japanese’ essay ‘Progressive Simplification’, where he talks about this at the sentence level:
As we move along through a Japanese sentence, we find that smaller units turn out to be parts of larger units. This does not make things more complicated, but rather the reverse: the sentence reduces itself to larger and simpler units as it goes along. It gets progressively simpler, rather than more complicated. No matter how long it is, and no matter how many nouns and particles and whatnot it may contain, any modifying clause is just an adjective if it ends up modifying a noun or an adverb if it ends up modifying a verb.
The jist is that Japanese is written towards clarification and resolution at the end of a statement. This occurs at the sentence level, but also the textual level. Everything written down is providing detail and context to what comes after. This means that when you start a new book, you’re slammed with fresh context, which manifests in complex sentences and phrases which are all to put into context everything you’re about to read thereafter. By the time you hit the meat of a text, sentences are reduced to one or two particles, and a verb. Light novels often don’t even tag who’s speaking in a dialogue heavy scene, because you have enough context to know who said what.
To conclude, feeling lost and confused is entirely reasonable and less a sign of being illiterate in Japanese, and more a sign of not being used to Japanese writing conventions. This gets compounded by your level of Japanese, which just adds more places for you to get confused than a typical reader, but working your way through a text should slowly solidify things piece by piece for you, including, like you experienced with the narrator’s brother’s name, correcting mistakes you made in your initial reading.
Everyone else is right in saying that sticking it out will in fact make it easier over time.
Note: This is not to suggest that Japanese fiction doesn’t do In Media Res, it absolutely does, but when you get to the first breakpoint in the story, almost always the text will slow down to contextualise everything for you.
Note the second: Putting it like this makes it sound like this is some Japanese-only phenomenon. It isn’t, all languages do this at a textual level, we’re all familiar with the Film Noir trope of the narrator taking his time to describe everything happening in front of him, but Japanese narrative style relies on it more heavily than most mainstream authors in English could afford to. As Rubin puts it
Anticipation is crucial for the efficient functioning of all languages. … [it is] not a feature unique to Japanese, though Japanese seems to tolerate more suspense than English as it moves along, leaving more judgements and distinctions suspended in anticipation of later clarification, than English would allow.
Like a getting hit in the face with a grouper.