Моя молодша сестра вже вища за мене, хоча вона молодша.

Because they both describe сестра, which is a feminine noun.

In Ukrainian, adjectives and possessives agree with the noun in gender, number, and case:

• сестра = feminine singular
• моя = my in feminine singular
• молодша = younger in feminine singular

So моя молодша сестра means my younger sister.

It means younger sister in terms of age.

That is the normal Ukrainian way to talk about a sibling who is younger than you:

• молодший брат = younger brother
• молодша сестра = younger sister

If you wanted to say youngest sister, that would be наймолодша сестра.

In the present tense, Ukrainian usually omits the verb бути (to be) in sentences like this.

So:

• сестра вже вища за мене literally looks like sister already taller than me
• but it means my sister is already taller than me

And:

• вона молодша = she is younger

This is completely normal in Ukrainian.

Вища is the feminine form of the comparative adjective вищий, meaning taller / higher.

It comes from високий = tall / high.

So the pattern is:

• високий = tall
• вищий = taller
• вища = taller (feminine)

It is feminine here because it refers to сестра / вона.

Ukrainian often makes comparisons with:

• a comparative adjective
  • за
    • the thing being compared

So:

• вища за мене = taller than me

This is a very common pattern:

• старший за мене = older than me
• швидший за мене = faster than me

Another possible way to say than is ніж:

• вища, ніж я
• вища за мене

Both are possible, but за мене is very common and natural.

Because after за in this comparison pattern, Ukrainian uses the object form of the pronoun, not the basic subject form.

So:

• я = I
• мене = me

That is why you get:

• за мене = than me

This is similar to how English often says than me, even though some formal grammar traditions prefer than I.

Вже means already.

It adds the idea that this change has happened by now or sooner than expected:

• Моя молодша сестра вже вища за мене
  = My younger sister is already taller than me

Without вже, the sentence would just state the fact more neutrally:

• Моя молодша сестра вища за мене
  = My younger sister is taller than me

Хоча means although / though.

It introduces a subordinate clause:

• ..., хоча вона молодша.
• ..., although she is younger.

The comma is used because хоча starts a subordinate clause. That punctuation is standard in Ukrainian.

Because the sentence is making a contrast:

• she is taller
• even though she is younger

So the repeated молодша is not redundant; it is the key contrast in the sentence.

The whole idea is:

• My younger sister is already taller than me, although she is younger.

The first молодша is part of my younger sister.
The second молодша is the predicate: she is younger.

Yes, Ukrainian can often omit subject pronouns when the meaning is clear.

So хоча молодша might be understandable in the right context, but хоча вона молодша sounds clearer and more complete here.

Including вона helps emphasize the contrast:

• although she is younger

So it is natural and stylistically good in this sentence.

Yes. Ukrainian word order is fairly flexible, and changes in order usually affect emphasis, not the basic meaning.

For example, the given sentence is neutral and natural:

• Моя молодша сестра вже вища за мене, хоча вона молодша.

If you move вже, you slightly change what is emphasized. But the original version is a very normal way to say it.

For learners, it is best to keep this word order until you become comfortable with Ukrainian emphasis patterns.