Вроде бы у сестры уже есть праздничное платье, но на будничный ужин она всё равно надевает мягкий кардиган.

Russian usually expresses possession with у + person in the genitive + есть.

So:

• у сестры = with/by the sister
• есть = there is / exists

Literally, it is something like By the sister there is..., but in natural English this means the sister has...

This is one of the most important Russian patterns:

• У меня есть книга. = I have a book.
• У него есть машина. = He has a car.

In your sentence:

• у сестры уже есть праздничное платье = the sister already has a festive dress

Вроде бы adds uncertainty or a soft, tentative tone. It often means:

• it seems
• apparently
• sort of
• it looks like
• as far as I can tell

So Вроде бы у сестры уже есть... suggests the speaker is not stating it in a hard, absolute way. It feels more like:

• It seems the sister already has...
• She appears to already have...

It is softer than a direct statement like У сестры уже есть...

Because after у in the possession pattern, the noun goes into the genitive case.

Base form:

• сестра = sister

Genitive singular:

• у сестры = of the sister / by the sister, which in this structure means the sister has

This is not specific to this word. Compare:

• у мамы = mom has
• у брата = the brother has
• у друга = the friend has

So сестры is there because у requires the genitive in this construction.

Because платье is a neuter singular noun, and the adjective must agree with it.

• праздничный = festive, holiday, dressy
• платье = dress

Since платье is neuter singular, the adjective becomes:

• праздничное платье

Russian adjectives change to match the noun in:

• gender
• number
• case

For example:

• праздничный ужин = festive dinner (masculine)
• праздничная рубашка = festive shirt/blouse (feminine)
• праздничное платье = festive dress (neuter)

These two adjectives are opposites in this sentence.

• праздничный = festive, holiday-related, special-occasion
• будничный = everyday, ordinary, routine, weekday-like

So:

• праздничное платье = a special or festive dress
• будничный ужин = an ordinary, non-special dinner

The contrast is important: she has a dress for special occasions, but for an everyday dinner she still puts on a soft cardigan.

Here на means something like:

• for
• to
• for the occasion of

Russian often uses на + accusative for events or occasions:

• на ужин = for dinner / to dinner
• на концерт = to a concert
• на свадьбу = to a wedding
• на праздник = for the holiday/celebration

So на будничный ужин means:

• for an ordinary dinner
• to an everyday dinner

It is not about physical location here. It is about the occasion.

Because ужин is a masculine singular noun, and after на in this meaning, it takes the accusative case.

Now, for an inanimate masculine noun, the accusative looks the same as the nominative:

• nominative: будничный ужин
• accusative: на будничный ужин

So the form stays будничный.

The same thing happens with кардиган later in the sentence:

• мягкий кардиган
• она надевает мягкий кардиган

Since кардиган is also masculine and inanimate, the accusative looks the same as the nominative.

Всё равно means:

• all the same
• anyway
• still
• nevertheless

It shows that what happens in the second part goes against what you might expect from the first part.

So the logic is:

• She already has a festive dress,
• but she still / anyway puts on a soft cardigan.

It adds a sense of despite that.

Very common examples:

• Я устал, но всё равно пойду. = I’m tired, but I’ll go anyway.
• Он знает, но всё равно спрашивает. = He knows, but he still asks.

Because надевать / надеть means to put on, while носить means to wear in the sense of have on / wear regularly.

So:

• она надевает кардиган = she puts on a cardigan
• она носит кардиган = she wears a cardigan

In this sentence, the focus is on the action of dressing for the dinner, so надевать is a better choice.

Also, надевает is imperfective, which is natural here for a repeated, usual, or descriptive action.

Compare:

• Она надевает мягкий кардиган. = she puts on a soft cardigan
• Она носит мягкий кардиган. = she wears a soft cardigan / is in the habit of wearing one

Because it is the direct object of надевает, so it is in the accusative case.

But кардиган is:

• masculine
• singular
• inanimate

For masculine inanimate nouns, the accusative is the same as the nominative:

• nominative: кардиган
• accusative: кардиган

So:

• она надевает мягкий кардиган

If it were an animate masculine noun, the accusative would usually match the genitive instead.

Yes, Russian can often omit subject pronouns when the verb already makes the subject clear.

So in theory, you could say:

• ...но на будничный ужин всё равно надевает мягкий кардиган.

However, она is very natural here because it helps:

1. keep the sentence clear,
2. mark the subject again after the conjunction но,
3. create a slight contrast or emphasis.

So она is not strictly necessary, but it sounds smoother and clearer in this sentence.

Russian word order is flexible, but the version here is natural and expressive.

The sentence is:

• Вроде бы у сестры уже есть праздничное платье, но на будничный ужин она всё равно надевает мягкий кардиган.

A rough breakdown:

• Вроде бы = softening / uncertainty at the start
• у сестры уже есть праздничное платье = background fact
• но = contrast
• на будничный ужин = setting/occasion
• она всё равно надевает мягкий кардиган = main contrasting action

Russian often places adverbs and context words where they sound most informative or contrastive, not always where English would put them.

For example, всё равно sits right before the verb phrase to highlight she still puts on the cardigan.

So the word order is doing more than grammar; it is also shaping emphasis and tone.