Learn Hardcore Russian: Мой брат — фанат этой команды, а сестра болеет за другую. - My brother is a fan of this team, and my sister supports another one.

Мой брат — фанат этой команды. (most common, stylistically neutral)
Мой брат фанат этой команды. (informal; often used in speech, without the dash)

Why is it этой команды (genitive) and not эта команда (nominative)?

The noun фанат (fan) usually takes a genitive to show what someone is a fan of:

фанат чего? — этой команды (of this team)

So we get:

Мой брат — фанат этой команды.
My brother is a fan of this team.

Some common patterns with genitive like this:

любитель чая — a lover of tea
ценитель музыки — a connoisseur of music
фанат футбола — a fan of football

Using nominative (эта команда) here would be ungrammatical:
✗ Мой брат — фанат эта команда — wrong.

What does болеет mean here? I thought болеть means “to be ill”.

The verb болеть has two main meanings, depending on the construction:

To be ill, to be sick
- Он болеет гриппом. — He is ill with the flu.
To support / root for (a team), to be a fan while watching sports
- Она болеет за другую (команду). — She supports another (team).

This “support” meaning appears when болеть is used with the preposition за + accusative:

болеть за кого? что? — to support whom? what?

So:

сестра болеет за другую = my sister roots for another team (not “my sister is sick for another one”).

Why is it за другую, and what exactly is другую here?

Другая is the feminine form of другой (other / another). In the sentence:

…а сестра болеет за другую.

the full phrase is за другую команду (for another team), but команду is omitted because it’s obvious from context. Russian often drops repeated nouns like this.

Другая becomes другую because:

за (in this meaning = for, in support of) takes the accusative.
команда is feminine.
Feminine accusative singular of другая is другую.

So:

болеть за другую (команду) — to support another (team).

Why do we have мой брат, but just сестра without моя?

Both versions are grammatically fine:

Мой брат — фанат этой команды, а моя сестра болеет за другую.
Мой брат — фанат этой команды, а сестра болеет за другую.

The author simply avoids repeating мой/моя: in Russian, once it’s clear we’re talking about my family, later mentions of close relatives often drop the possessive:

Мой папа врач, а мама учительница.

It still clearly means my sister from context. Using моя сестра would just sound a bit more explicit or slightly heavier, but not wrong.

Why is the conjunction а used instead of и between the two parts?

Both а and и can be translated as and, but they’re used differently:

и = and (simply adds information, no contrast)
а = and / whereas / but (marks a contrast or comparison)

Here we’re contrasting the brother’s and sister’s loyalties:

Мой брат — фанат этой команды, а сестра болеет за другую.
My brother is a fan of this team, whereas my sister supports another (one).

Using и would sound weaker and less natural here, because the point is the difference between the brother and sister.

Can I say Мой брат есть фанат этой команды or Мой брат является фанатом этой команды?

There are three main ways to say this:

Мой брат — фанат этой команды.
- Most natural and common.
Мой брат фанат этой команды.
- Informal, common in speech.
Мой брат является фанатом этой команды.
- Correct, but more formal / bookish (often in written or official style).

Мой брат есть фанат этой команды is grammatically possible but sounds unnatural in modern Russian, unless you are making some very strong emphasis or using archaic style. In everyday language, you almost never use есть as “is” in the present tense.

What grammatical case is мой брат and фанат in? Are they the same?

Yes, both are in the nominative case:

Мой брат — nominative (subject)
фанат — nominative (predicative complement)

In sentences like X — Y (X is Y), Russian typically uses:

Subject in nominative
- predicative noun also in nominative

So:

Мой брат — фанат (кто?) — nominative
Она — студентка. — She is a student.
Он — учитель. — He is a teacher.

What tense and aspect is болеет, and how would I say this in the past or future?

Болеет is:

Present tense
3rd person singular
Imperfective aspect of болеть

So:

сестра болеет за другую — my sister supports another (team) (now / generally)

Other forms with the “support a team” meaning:

Past:
- Она болела за эту команду. — She supported this team.
Future:
Imperfective: Она будет болеть за эту команду. — She will (be) support(ing) this team.
There is no separate perfective form for this meaning; болеть за is normally used in the imperfective.

Could the word order around болеет за другую be changed?

Slightly, yes, but with different emphasis:

Сестра болеет за другую. — neutral.
Сестра за другую болеет. — puts a bit more emphasis on за другую (on “the other team”).
За другую болеет сестра. — emphasizes сестра (as opposed to someone else).

All are grammatically correct. The original word order is the most neutral and typical in everyday speech.

Is there any difference between болеть за and using фанат when talking about sports?

They are related but not identical:

быть фанатом команды
- Describes a general identity / long-term attitude.
- Мой брат — фанат этой команды. — My brother is a fan of this team.
болеть за команду
- Describes actively supporting, especially while watching a match (cheering, emotionally invested).
- Сестра болеет за другую. — My sister supports another (team), usually in the context of watching games.

You can be:

фанат команды, even when no game is currently happening.
болеть за команду during a match, whether or not you’d call yourself a “fanatic” (фанат).