Моя сестра вибирає контракт.

Because моя has to agree with сестра.

In Ukrainian, possessive words like my change form depending on the noun they go with:

• мій = masculine singular
• моя = feminine singular
• моє = neuter singular
• мої = plural

Since сестра is a feminine singular noun, you use моя:

• моя сестра = my sister

So the form of my matches the gender, number, and case of the noun, not the speaker.

Сестра is in the nominative case because it is the subject of the sentence — the person doing the action.

In Моя сестра вибирає контракт, the subject is моя сестра, because she is the one choosing.

So:

• моя сестра = nominative
• контракт = accusative object of the verb

Вибирає is the 3rd person singular present tense form of вибирати.

That means it matches she/he/it:

• я вибираю = I choose / am choosing
• ти вибираєш = you choose
• вона вибирає = she chooses / is choosing

Because the subject is моя сестра = my sister, the verb must be вибирає.

This present tense can often mean either:

• chooses (habitual or general)
• is choosing (right now)

The exact English translation depends on context.

It actually is in the accusative case, but for many inanimate masculine nouns, the accusative looks exactly like the nominative.

So:

• nominative: контракт
• accusative: контракт

That is why the form does not visibly change.

This is very common in Ukrainian. Compare:

• Я бачу контракт. = I see a contract.
• Контракт лежить на столі. = The contract is lying on the table.

In both places, the form is контракт.

A useful contrast:

• inanimate masculine: accusative = nominative
• animate masculine: accusative usually = genitive

For example:

• брат = brother
• Я бачу брата. = I see my brother

Here the form does change, because brother is animate.

Ukrainian does not have articles.

So контракт can mean:

• a contract
• the contract

The listener understands which one is meant from context.

That is normal in Ukrainian. You do not need to add a separate word for a or the.

No, Ukrainian word order is fairly flexible.

The most neutral order here is:

• Моя сестра вибирає контракт.

This is a straightforward subject–verb–object order.

But other orders are possible for emphasis, contrast, or style, for example:

• Контракт вибирає моя сестра.
• Моя сестра контракт вибирає.

These do not necessarily change the basic meaning, but they can change what sounds emphasized.

For a learner, the safest default is:

• subject + verb + object

So this sentence is a very natural neutral order.

Because Ukrainian often does not need an extra subject pronoun when the subject is already clear.

Here, моя сестра already tells you who is doing the action, so adding вона would usually be unnecessary.

Compare:

• Моя сестра вибирає контракт. = My sister is choosing a contract.
• Вона вибирає контракт. = She is choosing a contract.

If you already say моя сестра, you normally do not also say вона, unless you want special emphasis.

A helpful stress marking is:

• моя́ сестра́ вибира́є контра́кт

A rough pronunciation guide for an English speaker is:

• mo-YA ses-TRA vy-by-RA-ye kon-TRAKT

A few notes:

• я in моя sounds like ya
• є in вибирає gives a ye / e-type sound in this ending
• the stress is important in Ukrainian and is not always predictable, so it is worth learning it with each word

They are very close, and in many contexts both can be translated as chooses or selects.

Very roughly:

• вибирати / вибрати often feels like everyday choosing/selecting
• обирати / обрати can also mean choose, and is often used in broader or more formal contexts, including electing someone

In many ordinary situations, both verbs may be possible, but the exact preference depends on context and style.

So in this sentence, вибирає is a normal and natural verb for is choosing / chooses.