Мой брат — самый сильный спортсмен в команде.

In Russian, when two nouns are linked in a present-tense sentence with the meaning X is Y, the verb to be is usually omitted, and a dash is often used in writing:

Мой брат — спортсмен. = My brother is an athlete.

So in Мой брат — самый сильный спортсмен в команде, the dash helps mark the link between the subject мой брат and the description that follows.

In casual writing, people sometimes leave the dash out, but in standard written Russian it is normal here.

Russian usually leaves out the present-tense verb to be.

So:

• Мой брат — спортсмен literally looks like My brother — athlete
• but it means My brother is an athlete

This is completely normal in Russian.
If you want past or future, then forms of быть appear:

• Мой брат был спортсменом. = My brother was an athlete.
• Мой брат будет спортсменом. = My brother will be an athlete.

Because брат is a masculine noun. Russian possessive words agree with the gender, number, and case of the noun they describe.

So:

• мой брат = my brother
• моя сестра = my sister
• моё место = my seat/place
• мои друзья = my friends

Since брат is masculine singular nominative, мой must also be masculine singular nominative.

Самый сильный means the strongest.

Russian often forms the regular superlative with:

самый + adjective

So:

• сильный = strong
• самый сильный = strongest
• красивый = beautiful
• самый красивый = most beautiful

This is one of the most common and straightforward ways to make a superlative in Russian.

Yes. Сильнейший also means strongest or very strong / the strongest, depending on context.

So you could say:

Мой брат — сильнейший спортсмен в команде.

That is grammatical, but самый сильный is often clearer and more neutral for learners.
In many situations:

• самый сильный = the most standard, transparent form
• сильнейший = a more compact literary-style superlative, sometimes a bit stronger in tone

Here спортсмен is a predicate noun in a present-tense sentence with no expressed verb to be, and Russian commonly uses the nominative in this kind of structure:

Мой брат — спортсмен.

So in your sentence:

Мой брат — самый сильный спортсмен в команде.

the noun спортсмен stays in the nominative.

You often see the instrumental case when there is an explicit verb such as быть, стать, or являться:

• Мой брат был спортсменом. = My brother was an athlete.
• Мой брат стал спортсменом. = My brother became an athlete.
• Мой брат является спортсменом. = My brother is an athlete.
  (more formal)

Because they both agree with спортсмен, which is masculine singular nominative.

In Russian, adjectives and adjective-like words must match the noun they describe. Here:

• самый describes спортсмен
• сильный also describes спортсмен

Since спортсмен is masculine singular nominative, both words take masculine singular nominative endings:

• самый
• сильный
• спортсмен

Because after в when it means in, Russian uses the prepositional case.

The noun команда changes like this:

• nominative: команда
• prepositional: в команде

So:

• в команде = in the team / on the team

This phrase tells us the group within which he is the strongest athlete.

This is just a difference in how the two languages express the idea.

English says on the team, but Russian normally says в команде, literally in the team.

This is very common: prepositions do not always match directly between English and Russian.
So it is best to learn в команде as the normal Russian expression.

Брат is in the nominative case.

It is the subject of the sentence: the sentence is about my brother, so брат appears in its basic dictionary form.

Also, мой matches it in nominative masculine singular:

• мой
• брат

Yes, Russian word order is more flexible than English word order.

The neutral version is:

Мой брат — самый сильный спортсмен в команде.

But other orders are possible for emphasis, for example:

В команде мой брат — самый сильный спортсмен.
This emphasizes in the team.

Самый сильный спортсмен в команде — мой брат.
This emphasizes that the strongest athlete on the team is my brother.

The original sentence is the most natural if you are simply stating the fact.

Yes, спортсмен is masculine.

For a woman, you would normally use спортсменка:

• Моя сестра — самая сильная спортсменка в команде.

Notice how several words change:

• моя because сестра is feminine
• самая сильная because спортсменка is feminine
• спортсменка because the person is female

A helpful stress guide is:

Мой брат — са́мый си́льный спортсме́н в кома́нде.

Key stresses:

• са́мый
• си́льный
• спортсме́н
• кома́нде

Stress is important in Russian because it is not always predictable and can affect pronunciation quite a lot.