sono yukata ha mitame hodo omokunakute, imouto ni mo yoku niatte imasita.

その means that and points to something close to the listener, already mentioned, or identifiable from context.

The basic contrast is:

• この = this
• その = that
• あの = that over there

So その浴衣 means that yukata. It often sounds like the speaker is referring to a specific yukata that both people already know about.

は marks 浴衣 as the topic of the sentence.

So その浴衣は means something like:

• As for that yukata, ...
• That yukata, ...

This sets up the yukata as the thing being talked about. After that, the sentence gives two comments about it:

1. it was not as heavy as it looked
2. it suited the younger sister well

Using は is very natural here because the sentence is describing the yukata.

見た目 means appearance, the way something looks, or outward impression.

In this sentence, 見た目 refers to how heavy the yukata seemed based on appearance. So 見た目ほど leads into the idea of:

• as much as it looks
• as much as its appearance suggests

A very natural English equivalent here is as it looks.

Here, ほど is used in a comparison pattern:

• A ほど B ない = not as B as A

So:

• 見た目ほど重くない

means:

• not as heavy as it looks
• literally, not heavy to the extent that its appearance suggests

This is a very common grammar pattern. For example:

• 思ったほど難しくない = not as difficult as I thought
• 見たほど大きくない = not as big as it looked

重い is an i-adjective, and its negative form is 重くない.

Then 重くなくて is the te-form-like connective of the negative adjective, used to link to the next clause.

Breakdown:

• 重い = heavy
• 重くない = not heavy
• 重くなくて = not heavy, and ... / because it was not heavy ...

In this sentence, it connects naturally to the next part:

• 見た目ほど重くなくて、妹にもよく似合っていました。

So the sentence gives two connected observations about the yukata.

It can suggest more than one relationship, depending on context.

In 重くなくて、妹にもよく似合っていました, なくて mainly works as a connector, similar to:

• and
• and also
• sometimes loosely because it wasn’t...

Here, the safest understanding is just that it links two descriptions:

• it was not as heavy as it looked,
• and it suited the younger sister well.

Japanese often leaves the exact logical connection slightly unstated when the flow is clear from context.

The verb 似合う means to suit or to look good on, and the person who something suits is marked with に.

So:

• 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 feels like the subject of the idea. In Japanese, the clothing or item is often the thing being discussed, and the person is marked with に.

Examples:

• この色は私に似合う = This color suits me.
• その帽子は彼に似合う = That hat suits him.

も means also, too, or even.

So 妹にも means:

• also on the younger sister
• it suited the younger sister too

This suggests some kind of added idea from the broader context, such as:

• it was not only nice in general, but it also looked good on the younger sister
• maybe it suited someone else too
• or the speaker is adding another positive point

The full nuance depends on context, but も definitely adds the sense of also/too.

Here, よく does not mean often. It means well.

So:

• よく似合う = suit well / look good on someone

This is a very common use of よく as the adverb form of いい.

Compare:

• よく行く = go often
• よく似合う = suit well
• よくできました = well done

So in this sentence, よく describes how well the yukata suited the younger sister.

似合っていました comes from 似合っている, which describes a state: something is suiting someone, or looks good on someone.

Since 似合う often describes an ongoing appearance/state, ている is very natural.

So:

• 似合っている = is suiting / looks good on
• 似合っていました = was suiting / looked good on

This often sounds more natural than 似合いました, because the sentence is describing how the yukata looked on the younger sister at that time.

In other words, 似合っていました focuses on the visible state or impression.

The past tense shows that the speaker is talking about a specific situation in the past.

For example, the speaker may be remembering the time when the younger sister wore the yukata. So:

• 似合っています = it suits her / it is looking good on her
• 似合っていました = it suited her / it was looking good on her

The same past-time reference also affects the earlier part of the sentence implicitly. Even though 重くなくて is not marked for past by itself, the whole sentence is understood as describing that past situation.

Not exactly. They are related, but the nuance is different.

• 見た目ほど重くない = not as heavy as it looks
• 見た目より軽い = lighter than it looks

Both can lead to a similar overall meaning, but the grammar is different:

• ほど ... ない = not as ... as ...
• より = than

So 見た目ほど重くない is the natural pattern used here.

A helpful breakdown is:

• その浴衣は = as for that yukata
• 見た目ほど = as its appearance suggests / as it looks
• 重くなくて = it was not that heavy, and ...
• 妹にも = also on the younger sister
• よく似合っていました = it looked very good / suited her well

So the sentence is basically:

• As for that yukata, it wasn’t as heavy as it looked, and it also suited my younger sister well.

It is one topic followed by two descriptive comments joined together naturally.