Mi hermana quiere una funda nueva para el edredón porque la otra ya está vieja.

Mi is the normal possessive adjective meaning my, and it goes before the noun: mi hermana.

• mi hermana = my sister
• la hermana de mí is not natural Spanish
• mía is a possessive pronoun/adjective used differently, as in la hermana mía or simply la mía, but that is not the basic, neutral way to say my sister

So for straightforward possession before a noun, Spanish uses mi.

Because the subject is mi hermana, which is she / my sister, so the verb querer has to be conjugated in the third person singular of the present tense:

• yo quiero = I want
• tú quieres = you want
• él/ella quiere = he/she wants

So:

• Mi hermana quiere... = My sister wants...

Funda means a cover or case that goes over something.

Common examples:

• funda de almohada = pillowcase
• funda de móvil = phone case
• funda de cojín = cushion cover

Here, funda ... para el edredón refers to a duvet cover or cover for the duvet.

In Spanish, descriptive adjectives usually come after the noun, so:

• una funda nueva = a new cover

That is the most neutral word order.

If you put the adjective before the noun, una nueva funda, it can sound a bit more like another/new replacement cover, with slightly more emphasis on the idea of newness as a change rather than simply describing it.

So una funda nueva is very natural here.

Para here shows purpose or intended use:

• una funda para el edredón = a cover for the duvet

In other words, it is a cover meant for the duvet.

A learner may also hear more idiomatic expressions such as:

• funda de edredón
• in Spain, very often funda nórdica

But para el edredón is perfectly understandable and clearly expresses what the cover is for.

Because edredón is a masculine singular noun, so its definite article is el:

• el edredón

After para + el, Spanish contracts them:

• para el = for the
• unlike a + el, which becomes al

So para el edredón simply means for the duvet.

Because porque is the conjunction meaning because.

• porque = because
• por qué = why

So:

• ...porque la otra ya está vieja = ...because the other one is already old/worn out

This is a very common spelling distinction in Spanish.

La otra means the other one.

The noun funda is omitted because it is already understood from the previous part of the sentence. Spanish often avoids repeating a noun when it is obvious.

So:

• la otra = the other one
• more fully: la otra funda

It uses la because funda is feminine.

This is a very useful ser vs estar point.

Here, está vieja suggests the cover is worn out, showing age, or has become old from use. That fits the idea that she wants a new one because the current one is no longer in good condition.

• estar viejo/a often suggests a state or condition something is in
• ser viejo/a more often describes something as old by age or nature

For an object that has become shabby or worn, está vieja sounds very natural.

Ya here means already, by now, or at this point.

It adds the idea that the condition has already happened or is now clearly true:

• está vieja = it is old / worn out
• ya está vieja = it is already old / worn out now

So it strengthens the reason for needing a replacement.

Usually vieja is better here.

• vieja = old, worn, no longer in good condition
• antigua = old in age, ancient, antique, from an earlier period

For a duvet cover that needs replacing, vieja is the natural choice because it suggests used and worn. Antigua would sound odd unless you were talking about it as an old-fashioned or antique object.

Because the direct object is a thing, not a specific person.

• quiere una funda = wants a cover

The personal a is normally used before specific people (and sometimes personified animals):

• Quiere a su hermana = He/She loves his/her sister
• Busca a su madre = He/She is looking for his/her mother

But with an object like una funda, there is no a.

In Spain, edredón usually refers to a thick bed covering, often something like a duvet or quilted comforter.

Depending on region and context, you may also hear:

• nórdico = duvet / duvet insert
• funda nórdica = duvet cover

So a learner in Spain should know that bed-linen vocabulary can vary a bit, but edredón is a very normal word and will be understood.