ani no gakkou ha kyonen no kessyou de maketa node, kotosi ha katitai to minna ga itte iru.

It means my older brother’s school.

Here, の links two nouns and shows a relationship between them. In this case, it is basically possession/association:

• 兄 = older brother
• 兄の学校 = older brother’s school

In natural English, this would usually be understood as my older brother’s school, because plain 兄 normally refers to the speaker’s own older brother when talking to someone outside the family.

They are both doing the same basic job: connecting one noun to another.

• 兄の学校 = my older brother’s school
• 去年の決勝 = last year’s final / the final last year

So の is very often like English ’s or of, depending on context.

は marks the topic of the sentence.

So 兄の学校は means something like:

• As for my older brother’s school...
• Speaking of my older brother’s school...

The sentence is then telling you something about that topic.

This does not necessarily mean 学校 is the grammatical subject of every later verb. Japanese often sets a topic first, and then the rest of the sentence gives information related to that topic.

去年の決勝で means in/at last year’s final.

• 去年 = last year
• 決勝 = final, championship match, final round
• で marks the place or event where something happened

So 決勝で負けた means lost in the final.

Here, で is natural because the losing happened at/in that event.

ので means because or since.

So:

• 負けたので = because they lost / since they lost

In this sentence, it explains the reason for what comes next: because they lost last year, everyone is now saying they want to win this year.

Compared with から, ので often sounds a bit more explanatory, softer, or more objective.

Because the losing happened in last year’s final.

• 去年の決勝で負けた = lost in last year’s final

That event is completed, so the past tense is exactly what you would expect.

Because たい attaches to the verb stem, not the dictionary form.

For 勝つ:

• dictionary form: 勝つ
• stem / ます-stem: 勝ち
• たい form: 勝ちたい

So:

• 勝ちたい = want to win

This is a very common pattern:

• 食べる → 食べたい
• 行く → 行きたい
• 勝つ → 勝ちたい

It is the people referred to by みんな who want to win.

A very important point is that 勝ちたい is inside a quote before と言っている, so it reflects what those people are saying:

• 今年は勝ちたい = We want to win this year / I want to win this year

Then:

• みんなが言っている = everyone is saying

So the full idea is that everyone is saying they want to win this year.

It is not that the school building itself has a desire. The implied people are probably the team members, students, or people connected with the school.

と marks the content of what is being said or thought.

So:

• 勝ちたい = want to win
• 勝ちたいと = that they want to win
• 勝ちたいとみんなが言っている = everyone is saying that they want to win

This と is the standard quotation/content particle used with verbs like:

• 言う = say
• 思う = think
• 聞く = hear/ask

が marks みんな as the subject of 言っている.

So the sentence is focusing on who is saying it:

• みんなが言っている = everyone is saying it

If you used は, it would give more of a topic/contrast feeling, like as for everyone, they are saying... That is possible in some contexts, but が is the straightforward choice here.

言っている shows an ongoing or current situation.

So it means something like:

• everyone is saying
• everyone keeps saying
• everyone has been saying

In context, this suggests a present state: this is what everyone is currently saying this year, probably as a continuing attitude or repeated comment.

If it were 言った, it would sound more like said at one specific past time.

In this kind of sports context, 学校 often refers to the school as a representative group, especially the team.

So 兄の学校は去年の決勝で負けた naturally means something like:

• My older brother’s school lost in last year’s final
• more naturally in English: My older brother’s school team lost in last year’s final

Japanese often uses the larger group name this way, just like English sometimes says Our school won.

Here は gives a topic/contrast feeling:

• 去年 = last year
• 今年は = as for this year / this year, though

It contrasts this year with last year:

• last year they lost
• this year they want to win

So this は helps highlight the contrast between 去年 and 今年.

Yes, Japanese often leaves things unstated when they are clear from context.

Here, 今年は勝ちたい does not spell out exactly what they want to win, but the context already gives it to us:

• they lost in last year’s final
• so this year they want to win

The implied meaning is something like:

• This year, we want to win the final
• This year, we want to win the championship
• This year, we want to win

Japanese frequently omits information that would sound repetitive.

A natural breakdown is:

• 兄の学校は = As for my older brother’s school,
• 去年の決勝で負けたので、 = because they lost in last year’s final,
• 今年は勝ちたい = this year they want to win,
• とみんなが言っている。 = everyone is saying that.

So the logic is:

1. set the topic: my older brother’s school
2. give the reason: they lost last year
3. give the quoted desire: want to win this year
4. say who is expressing that desire: everyone

That kind of long chained structure is very normal in Japanese.