Nejen moje sestra, ale i moje maminka chtějí zítra poslat balík.

It means not only ... but also ....

So:

• Nejen moje sestra = Not only my sister
• ale i moje maminka = but also my mother

In this sentence, the structure links two parallel subjects:

• moje sestra
• moje maminka

Czech uses this pair very much like English uses not only ... but also ....

A very common variant is nejen ..., ale také ....

Because Czech normally puts a comma before ale in the structure nejen ..., ale i/také ....

So the comma is not really because of sestra itself; it is there because of the conjunction pattern:

• Nejen moje sestra, ale i moje maminka ...

This is standard punctuation.

Because together they form a compound subject: two people are doing the wanting.

Even though each noun is singular on its own, the whole subject is plural:

• my sister + my mother = they

So Czech uses the plural verb:

• chtějí = they want

This is exactly the same logic as in English:

• Not only my sister but also my mother want/wants...?

In standard Czech here, the plural chtějí is the normal choice.

Yes, má sestra and má maminka are also grammatically correct.

Czech has two common possessive forms here:

• má = shorter form
• moje = longer form

Both can mean my.

In everyday speech, moje is very common and natural.
The shorter má can sound a bit more literary, compact, or sometimes more emphatic, depending on context.

So all of these are possible:

• moje sestra
• má sestra

In this sentence, moje sounds completely normal.

Not absolutely, but repeating it is very natural here.

• Nejen moje sestra, ale i moje maminka ...

This repetition helps because:

• it keeps the two parts nicely parallel
• it sounds balanced
• it avoids any ambiguity

You may also hear:

• Nejen moje sestra, ale i maminka ...

That can work if the meaning is clear from context. But repeating moje is often the safest and most elegant choice in this kind of contrastive structure.

Because Czech does not use a separate word like English to before the infinitive in this construction.

English:

• they want to send

Czech:

• chtějí poslat

After chtít (to want), Czech simply uses the infinitive directly:

• chtít poslat = to want to send
• chtějí poslat = they want to send

So poslat is already the infinitive form; no extra word is needed.

Because poslat is perfective, while posílat is imperfective.

Here the sentence talks about sending one package tomorrow as a single completed action. For that, Czech naturally prefers the perfective verb:

• poslat balík = to send a package / to send off a package

If you used posílat, it would sound more like:

• repeated sending
• habitual action
• focusing on the process rather than one completed act

So in this sentence, poslat is the natural choice.

Balík is the direct object of poslat, so it is in the accusative case.

However, balík is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular is identical to the nominative singular.

So:

• nominative: balík
• accusative: balík

That is why the form does not visibly change.

Yes, the word order can change.

Czech word order is more flexible than English, and different placements can shift emphasis slightly.

This sentence is a neutral, natural version:

• Nejen moje sestra, ale i moje maminka chtějí zítra poslat balík.

You could also say:

• Zítra chtějí nejen moje sestra, ale i moje maminka poslat balík.
• Nejen moje sestra, ale i moje maminka chtějí poslat balík zítra.

Those are possible, but they may sound less neutral or place emphasis differently.

In the original sentence, zítra sits in a very natural position.

Maminka is not as neutral or formal as matka.

It is a warm, everyday word, often closer in feeling to:

• mom
• mum
• sometimes even mummy depending on context

So:

• matka = more formal, official, or clinical
• maminka = affectionate, family-style, natural in everyday speech

In this sentence, maminka sounds personal and natural.

Yes. These are all common:

• nejen ..., ale i ...
• nejen ..., ale také ...
• nejen ..., ale taky ... (more colloquial)

So you could say:

• Nejen moje sestra, ale také moje maminka chtějí zítra poslat balík.

The meaning stays the same.
The original ale i is perfectly natural and standard.