Learn Hardcore Russian: Моя сестра моложе, чем мой брат.

The “is” is understood from the structure [subject] + [adjective], so no separate word is needed in the present tense. (In past and future you do use forms of быть: была, будет, etc.)

Why is it моя сестра, not something like моей сестры or мою сестру?

Моя сестра is in the nominative case because it’s the subject of the sentence.

моя – feminine singular nominative form of мой, agreeing with сестра
сестра – feminine singular nominative, “sister”

Other forms would be different cases and wrong here:

моей сестры – genitive (“of my sister”)
мою сестру – accusative (“my sister” as a direct object)

Since “my sister” is the one who is younger, it must be nominative: моя сестра.

What exactly is моложе grammatically? Why doesn’t it change for gender or number?

Моложе is the comparative degree of the adjective молодой (“young”).

молодой – positive degree (“young”)
моложе – comparative (“younger”)

Comparatives of this type (ending in -ее / -ей, like старше, лучше, хуже, ближе) are indeclinable: they do not change for gender, number, or case.

So you always say:

Он моложе. – He is younger.
Она моложе. – She is younger.
Они моложе. – They are younger.

The form моложе stays the same; only the subject changes.

Why is it моложе, not более молодая? Are both possible?

Both are grammatically possible, but моложе is the normal, natural way to say “younger”.

Моя сестра моложе, чем мой брат. – perfectly natural.
Моя сестра более молодая, чем мой брат. – understood, but heavier and usually unnecessary.

Russian prefers single-word comparatives like:

моложе (younger)
старше (older)
лучше (better)
хуже (worse)
выше (taller / higher), etc.

The более + adjective form usually appears:

when no short comparative exists,
or for stylistic/emphatic reasons (often in written or formal language).

What is the difference between моложе and младше? Which is more natural here?

Both can talk about being younger, but there is a nuance:

младше – from младший.
Commonly used for age differences between people, especially relatives:
- Она младше брата на три года. – She is three years younger than her brother.
моложе – from молодой.
More general: can describe age or appearance:
- Она выглядит моложе. – She looks younger.

In family age comparisons, many Russians feel младше is slightly more natural:

Моя сестра младше моего брата. – very idiomatic.

Your original sentence with моложе is still correct and will be understood as a simple age comparison; it just leans a bit more toward the neutral “is younger” rather than the specifically “is the younger sibling” nuance.

Why do we use чем here? Is it the same as “than”? Can I use как instead?

In this sentence, чем functions like English “than” in a comparison of inequality:

моложе, чем мой брат – younger than my brother

You cannot replace чем with как here:

✗ Моя сестра моложе, как мой брат. – incorrect.

Как can mean “as / like” in comparisons of similarity:

Она такая же высокая, как брат. – She is as tall as (her) brother.

So:

comparative + чем → “younger / older / better than …”
(такой же / столько же / не менее и т.п.) + как → “as … as …”

Why is it чем мой брат (nominative), not чем моего брата (genitive)? Are there alternatives?

After a comparative, Russian has two main patterns:

Comparative + чем + nominative
- Моя сестра моложе, чем мой брат.
- Он старше, чем я.
Comparative + genitive (without чем)
- Моя сестра моложе моего брата.
- Она младше брата.

So you can say:

Моя сестра моложе, чем мой брат.
Моя сестра моложе моего брата.

Both are correct; the meaning is the same.
The чем + nominative pattern is very common in speech and is often clearer for learners.
The genitive without чем is also very common and a bit more compact.

Why is there a comma before чем? Is it always required?

In your sentence it’s written as:

Моя сестра моложе, чем мой брат.

In modern usage, with simple comparisons like this, you will see it both with and without a comma:

Моя сестра моложе, чем мой брат.
Моя сестра моложе чем мой брат.

The logic:

When чем clearly introduces a separate clause (“than I thought”, “than he is”), a comma is normally used:
- Он моложе, чем я думал. – He is younger than I thought.
In short, everyday comparisons, the comma is often omitted, and this is very common in contemporary texts.

For learning purposes, you won’t be misunderstood either way. Follow the rule your teacher or textbook gives you, but be aware that in real texts you’ll see both variants.

Can I drop мой before брат and just say …чем брат or …моложе брата?

Yes, grammatically you can:

Моя сестра моложе брата.
Моя сестра моложе, чем брат.

However:

Without мой, it’s less explicit that this is “my brother.” It will usually be understood from context (siblings in the same family), but it could be ambiguous if other brothers are being discussed.
With мой брат, it’s crystal clear: my brother.

So:

Use мой брат when you want to be explicit:
- Моя сестра моложе, чем мой брат.
Use just брата/брат, when the context already makes it obvious whose brother it is:
- Сестра младше брата. – The sister is younger than the brother (within the same family, understood from context).

Can I shorten the sentence to just Моя сестра моложе?

Yes, that is possible:

Моя сестра моложе. – My sister is younger.

In this case, the comparison target is implied by context:

Earlier in the conversation someone mentioned another person:
- Ты старший? – Нет, моя сестра моложе.
  – Are you the older one? – No, my sister is younger.

By itself, Моя сестра моложе sounds unfinished if there is no context. Normally, you either:

name what you compare with:
- Моя сестра моложе брата.
- Моя сестра моложе, чем мой брат.
or make clear from the previous sentence who you’re comparing her to.