Ці близнюки дуже схожі, але в однієї є окуляри, а в другої довге волосся.

Ці is the plural form of цей and means these.

So:

• цей = this
• це = this/it, or the neuter singular form
• ці = these

Because близнюки is plural, the demonstrative has to be plural too: ці близнюки = these twins.

Близнюки is a very common plural word for twins in general. It can be used even when the twins are female, especially in neutral everyday speech.

If you wanted to make the feminine meaning more explicit, you could also hear близнючки.

So:

• близнюки = twins, often general/neutral
• близнючки = female twins

The later forms однієї and другої show that the speaker is thinking of them as female individuals.

It is in the nominative plural, because it is the subject of the sentence.

• ці = nominative plural
• близнюки = nominative plural

They are the ones being described as дуже схожі.

In Ukrainian, схожий / схожа / схожі can sometimes stand on its own when the comparison is obvious from context.

Here, Ці близнюки дуже схожі means These twins look very similar / look alike.
The idea is really similar to each other, so Ukrainian does not need to say it explicitly.

A fuller version could be something like:

• Ці близнюки дуже схожі одна на одну.

But in the original sentence, that extra part is simply understood.

They are related, but they are not used in exactly the same way.

• але often introduces a stronger contrast: but
• а often links two contrasting or compared facts: while / whereas / and

So the structure is roughly:

• These twins are very similar, but...
• one has glasses, while the other has long hair

That is why both conjunctions sound natural here:

• але sets up the main contrast with the first clause
• а contrasts the two twins with each other

This is a very common Ukrainian way to express possession.

The pattern is:

• у / в + genitive + є + noun

Literally, it is something like:

• at one there are glasses

But in natural English, that becomes:

• one has glasses

So в однієї є окуляри means one has glasses.

Ukrainian can also use мати:

• Одна має окуляри, а друга має довге волосся.

That is grammatical too. But the у/в + genitive + є pattern is extremely common and natural.

Because they refer to the two twins as female individuals.

These are feminine singular forms:

• одна = one, feminine
• друга = the second / the other, feminine

After у/в in this possession pattern, they appear in the genitive:

• в однієї
• в другої

If the twins were male, you would expect:

• в одного
• в другого

Here they mean:

• однієї = one of them
• другої = the other one / the second one

So другої does not necessarily mean a strict numbered second in an important sense. In a pair, it often simply means the other one.

Yes, in a sense. Ukrainian often leaves the noun out when it is obvious from context.

So:

• в однієї really means in/at one of the twins
• в другої means in/at the other twin

English does something similar with one and the other.
Ukrainian does not need to repeat близнюки or another noun here.

That is true in many sentences:

• Вона гарна. = She is pretty.

But є is often kept in existential or possession sentences, especially with the pattern у когось є щось.

So:

• в однієї є окуляри
• в другої є довге волосся would also be possible

The є here helps express there is / there are / has.

In casual speech, people sometimes omit it, but with possession this full pattern is very standard.

Because окуляри works like English glasses: it is normally a plural-only noun.

So even for one pair of glasses, Ukrainian usually says:

• окуляри

not a singular form in normal everyday usage.

That is why the sentence says:

• є окуляри = there are glasses / one has glasses

Because волосся in Ukrainian is usually treated as a collective noun and is grammatically neuter singular.

So the adjective must match it:

• довге волосся = long hair

Even though hair consists of many hairs, grammatically the noun behaves like a singular mass/collective noun.

That is why you get:

• довге = neuter singular
• волосся = neuter singular noun

Yes. У and в are often interchangeable in Ukrainian, and the choice is usually made for euphony, meaning what sounds smoother.

So both are possible:

• в однієї
• у однієї

The same is true in many other places in Ukrainian. Native speakers often choose the version that flows better with the surrounding sounds.

Yes, and that would slightly shift the nuance.

• в однієї є окуляри = one has glasses
• одна в окулярах = one is wearing glasses / one is in glasses

If the point is specifically about visible appearance right now, одна в окулярах can sound especially natural.
The original sentence is still completely fine; it just uses a possession structure rather than a clothing/appearance structure.