sono yukata ha aoi hana no moyou de, imouto ni yoku niatte imasita.

その means that and points to something close to the listener or something already known in the conversation.

So その浴衣 means that yukata.

Japanese has a three-way distinction:

• この = this (near the speaker)
• その = that (near the listener / already mentioned)
• あの = that over there (far from both)

Here, その identifies which yukata is being talked about.

浴衣 is read ゆかた and refers to a casual Japanese summer garment, similar to a light cotton kimono.

English translations often leave it as yukata rather than translating it fully, because it is a specific cultural item.

青い is an i-adjective, meaning blue.

When an i-adjective directly modifies a noun, it stays in its plain form:

• 青い花 = blue flowers

So here:

• 青い modifies 花
• 青い花 = blue flowers

Then that whole phrase connects to 模様:

• 青い花の模様 = a pattern of blue flowers / a blue-flower pattern

Using 青の花 would sound unnatural here if you simply want to say blue flowers.

の connects nouns.

• 花 = flower(s)
• 模様 = pattern, design

So 花の模様 literally means flower pattern or a pattern of flowers.

In this sentence:

• 青い花の模様 = a pattern of blue flowers

The の shows that the pattern is made up of, or characterized by, flowers.

Here, で is not the location particle. It is the continuative form of だ / a linking form used after a noun phrase.

So:

• 青い花の模様で、 = it had a blue-flower pattern, and...

It connects the first description to the second part of the sentence.

You can think of it as:

• Xで、Y = it is/was X, and Y

So the sentence is describing the yukata in two parts:

1. it had a blue flower pattern
2. it suited the younger sister well

は marks the topic of the sentence.

• その浴衣は = as for that yukata / that yukata...

The rest of the sentence gives information about that topic.

Even though English would often say The yukata..., Japanese uses は to show what the sentence is about, not necessarily the grammatical subject in the English sense.

似合う means to suit or to look good on.

The person whom something suits is marked with に:

• 妹に似合う = to suit the younger sister
• literally, to match the younger sister

This is different from English, where the person often becomes the subject:

• It suits my sister
• My sister looks good in it

In Japanese, the clothing/item is often the thing doing the suiting:

• その浴衣は妹に似合う
• That yukata suits my younger sister

Yes, in a sentence like this, 妹 normally means my younger sister.

Japanese often omits words like my when they are understood from context. So:

• 妹 = younger sister
• in context, usually my younger sister

Also, 妹 specifically means younger sister. An older sister would be 姉.

Here, よく means well, not often.

よく can have more than one meaning depending on context:

• よく行く = go often
• よく似合う = suit well / look very good on

So in this sentence:

• 妹によく似合っていました
• it suited my younger sister well

It describes degree or quality, not frequency.

似合っていました is the past form of 似合っている.

With verbs like 似合う, the -ている form often describes a state rather than an action in progress. So:

• 似合う = suits
• 似合っている = is suiting / looks good on
• 似合っていました = was looking good on / suited well

This sounds natural when describing how something looked at that time.

Compare:

• 似合いました can sound more like it suited her
• 似合っていました emphasizes the resulting state/appearance: it was looking good on her

In many cases, 似合っていました is the more natural choice for describing appearance in a scene or memory.

Yes. The topic introduced by その浴衣は continues through the sentence.

So the yukata is still what is being talked about:

• その浴衣は ... 妹に よく 似合っていました
• That yukata ... suited my younger sister well

Japanese often does not repeat the noun once the topic has been established.

Yes. That is a very natural English translation.

Literally, it is:

• a pattern of blue flowers

But in smoother English, you might say:

• a blue floral pattern
• a pattern of blue flowers
• a blue flower design

All of these capture the Japanese well, depending on style.

The comma helps show a pause between the two linked parts of the sentence:

• その浴衣は青い花の模様で、
• 妹によく似合っていました。

It is not absolutely required in every case, but it is very natural in writing because で is linking one description to another. The comma makes the sentence easier to read.