ane ha kuroi sukaato ni siroi syatu wo awaseruno ga zyouzu da.

姉 means (my) older sister.

In Japanese, 姉 is the plain word you use when talking about your own older sister to someone else.
By contrast, お姉さん is usually used when:

• referring respectfully to someone else’s older sister
• directly addressing your own older sister
• speaking in a softer or more polite way in some contexts

So in this sentence, 姉は is the normal way to say as for my older sister...

The particle は marks 姉 as the topic of the sentence.

So 姉は means something like:

• As for my older sister, ...
• My older sister ...

The sentence is then saying something about her: she is good at matching clothes.

Japanese often leaves out information that is obvious from context.

If you simply say 姉, it usually means my older sister unless the context suggests otherwise. So 私の is often unnecessary.

That is very natural in Japanese.

黒い and 白い are i-adjectives, so they can directly modify nouns:

• 黒いスカート = a black skirt
• 白いシャツ = a white shirt

You can also sometimes see forms like:

• 黒のスカート
• 白のシャツ

Those are possible too, especially in fashion or product-description contexts, but 黒い and 白い are very standard and natural here.

One small note: not all color words behave the same way. For example:

• 赤い, 青い, 白い, 黒い can act as true adjectives
• but many other colors often use ～の, like ピンクのシャツ

This is because of how 合わせる works in this sentence.

Here, X に Y を合わせる means:

• to match Y with X
• to pair Y with X

So:

• 黒いスカートに = with a black skirt
• 白いシャツを = a white shirt

In other words, the shirt is the thing being matched, so it takes を, and the skirt is the thing it is being matched with, so it takes に.

So the structure is:

• 黒いスカートに 白いシャツを 合わせる
• to match a white shirt with a black skirt

Here, 合わせる means to match or to coordinate.

In clothing contexts, it often means:

• to pair one item with another
• to choose clothes that go well together

So this is not just the literal sense of put together. It has the fashion/styling sense of coordinate.

The sentence presents it as:

• 黒いスカートに = with a black skirt
• 白いシャツを合わせる = match a white shirt

So literally, it is matching a white shirt with a black skirt.

In practice, the overall meaning is simply that she is good at coordinating those kinds of clothing combinations. The exact direction is less important than the combination itself.

The の turns the verb phrase into something noun-like.

So:

• 合わせる = to match
• 合わせるの = the act of matching / matching clothes

This allows the whole action to be treated as something that can be the subject of 上手だ.

A very literal breakdown would be:

• 黒いスカートに白いシャツを合わせるのが上手だ
• She is skilled at the act of matching a white shirt with a black skirt

This verb + の pattern is very common when talking about being good at, liking, disliking, or evaluating an action.

Because 上手 describes what someone is good at, and the thing they are good at is marked with が.

So:

• ～のが上手だ = to be good at doing ~

Examples:

• 料理をするのが上手だ = good at cooking
• 歌うのが上手だ = good at singing

Using は would change the nuance and usually sound contrastive, like emphasizing that particular activity as opposed to something else. In a neutral sentence, が is the normal choice.

上手 is a na-adjective (and historically noun-like in behavior).

That is why it works like this:

• 上手だ = is skillful / is good at
• 上手です = polite form
• 上手な人 = a skillful person

It is not an i-adjective, so you do not say 上手い when you mean this word.

Be careful: there is also うまい, which can mean skillful or good/tasty, but that is a different word and has a different nuance. In standard learner Japanese, 上手だ is the safer word for being good at something.

Because Japanese often uses this pattern:

• verb dictionary form + のが上手だ

This means to be good at doing that action.

So the sentence does not mean that the clothes themselves are skillful. It means your older sister has skill in the action of coordinating them.

When a verb comes before の in this kind of structure, the plain/dictionary form is normal.

So:

• 合わせるのが上手だ = is good at matching

This is just the standard grammar pattern.
You do not need to make the verb polite before の.

Yes, you could say:

• 黒いスカートに白いシャツを合わせることが上手だ

But that sounds less natural.

With action-based skills and everyday activities, verb + のが上手 is much more common and natural than ことが上手.

So 合わせるのが上手だ is the better choice here.

Yes, very much so.

Because the sentence uses clothing items plus 合わせる, it suggests:

• being good at coordinating outfits
• having a good sense of what clothes go well together

So the nuance is not only technically able to match, but also having good styling sense.

Yes.

• 上手だ = plain
• 上手です = polite

So the polite version of the whole sentence would be:

• 姉は黒いスカートに白いシャツを合わせるのが上手です。

The meaning stays the same; only the level of politeness changes.

Yes, it is natural.

It clearly means that the speaker’s older sister is good at pairing a white shirt with a black skirt, or more generally, good at coordinating clothes in that style.

Depending on context, a Japanese speaker might also say things like:

• 姉は服を合わせるのが上手だ。
  My older sister is good at coordinating clothes.

• 姉はコーディネートが上手だ。
  My older sister is good at styling outfits.

But the original sentence is grammatically fine and natural.