Моя двоюрідна сестра старша за мене, а племінниця ще ходить до школи.

Because сестра is a feminine singular noun, and Ukrainian possessives agree with the noun in gender, number, and case.

• мій брат = my brother
• моя сестра = my sister
• моє місто = my city
• мої друзі = my friends

Here, моя matches двоюрідна сестра.

In standard Ukrainian, a female cousin is very commonly called двоюрідна сестра, and a male cousin is двоюрідний брат.

Literally, it looks like second-degree sister/brother, but idiomatically it just means cousin.

There are borrowed words like кузина and кузен, but двоюрідна сестра / двоюрідний брат is the more neutral and standard everyday choice.

Because Ukrainian usually omits the present-tense copula (to be) in sentences like this.

So:

• Моя сестра старша за мене = My sister is older than me

A literal є is usually not needed here. This is very normal in Ukrainian.

Старша is the feminine singular form of старший, which means older / elder when talking about people.

It agrees with сестра, so you get:

• старший брат = older brother
• старша сестра = older sister

In this sentence, старша за мене means older than me.

After a comparative like старша in this pattern, Ukrainian often uses за + accusative:

• старша за мене = older than me
• молодший за тебе = younger than you

Here мене is the pronoun form used after за.
This is a very common Ukrainian way to say than me.

You may also see another pattern such as ніж:

• старша, ніж я

But старша за мене is very natural and common.

А often links two ideas with a contrast or comparison. In English, depending on context, it can sound like:

• and
• while
• whereas
• sometimes but

In this sentence, it feels like:

• My cousin is older than me, while my niece still goes to school.

So it is not a strong contradiction; it is more of a contrastive “and”.

Ukrainian often leaves out possessives when they are clear from context.

So племінниця here can naturally mean my niece if that is already understood.

If you want to be explicit, you can absolutely say:

• ...а моя племінниця ще ходить до школи.

Both are possible; the version without моя is just a bit lighter and more natural in context.

Here ще means still.

So:

• племінниця ще ходить до школи = my niece still goes to school / is still in school

In other contexts, ще can also mean yet, more, or else, but here still is the right sense.

Because ходить here means goes regularly / attends.

• ходить до школи = goes to school, attends school
• йде до школи = is going to school right now

So the sentence is about a habit or ongoing life situation, not one specific trip. That is why ходить is the natural choice.

The preposition до takes the genitive case, so:

• школа → до школи

That is why you see школи, not школа.

Also, ходити до школи is a very common expression meaning to attend school / to go to school regularly.

Because а is joining two clauses, and each clause has its own predicate:

• Моя двоюрідна сестра старша за мене
• племінниця ще ходить до школи

In Ukrainian, a comma is normally used before а in this kind of sentence.