Mi hermana empezó a deshacer la cama mientras yo intentaba no apilar más libros sobre la silla.

Mi hermana is the normal, natural way to say my sister in Spanish.

Spanish usually puts the possessive before the noun:

• mi hermana = my sister
• mi libro = my book

The longer form with mío/mía/míos/mías is usually used for emphasis or contrast:

• una hermana mía = a sister of mine
• la hermana mía sounds marked or literary in many contexts

So in an ordinary sentence, mi hermana is exactly what you would expect.

After empezar, Spanish normally uses a before another verb in the infinitive.

Pattern:

• empezar a + infinitive = to begin to + verb

Examples:

• Empezó a leer = She began to read
• Empezaron a hablar = They began to talk

So:

• empezó a deshacer la cama = she began to unmake the bed

This is a very common structure, and learners usually just need to memorize it as empezar a.

It literally means to undo the bed, but in natural English it usually means to unmake the bed or to pull the bed apart after it has been made.

It comes from:

• hacer la cama = to make the bed
• deshacer la cama = to unmake the bed

The verb deshacer often means to undo, to take apart, or to reverse what was done:

• deshacer un nudo = to untie a knot
• deshacer la maleta = to unpack a suitcase

So here it suggests she was taking the bedcovers apart rather than making the bed neat.

Because deshacer and deshacerse are different verbs.

• deshacer = to undo, unmake, take apart
• deshacerse de = to get rid of

Examples:

• Deshizo la cama = She unmade the bed
• Se deshizo de los libros = She got rid of the books

In your sentence, the meaning is about physically undoing the bed, so deshacer la cama is correct.

This is one of the most important tense contrasts in Spanish.

• empezó (preterite) presents the action as a completed event or a clear starting point
• intentaba (imperfect) presents the action as ongoing, in progress, or background

So the sentence gives this picture:

• Mi hermana empezó... = a specific event happened: she started
• mientras yo intentaba... = at the same time, I was in the middle of trying

In English, this is often like:

• My sister started... while I was trying...

Spanish uses this contrast a lot to show foreground vs background in past narration.

Yes, grammatically it could be, but it would change the feel.

• intentaba = I was trying / I kept trying
• intenté = I tried

With intentaba, the action sounds ongoing and simultaneous with what the sister was doing.

With intenté, it would sound more like a separate, completed attempt:

• Mi hermana empezó a deshacer la cama mientras yo intenté... sounds less natural because mientras often sets up overlapping actions, and the imperfect fits that better.

So intentaba is the more natural choice if the idea is while I was trying...

Mientras means while.

It introduces an action happening at the same time as another action:

• Mi hermana empezó... mientras yo intentaba...
• My sister started... while I was trying...

A very common pattern is:

• preterite for the main event
• imperfect for the background or ongoing action

So here:

• empezó = the event that happens
• intentaba = the ongoing action during that time

That combination is very typical and natural in Spanish storytelling.

Because intentar is followed directly by another verb in the infinitive.

Pattern:

• intentar + infinitive = to try to + verb

Examples:

• Intento dormir = I try to sleep
• Intentaba estudiar = I was trying to study

So:

• intentaba no apilar más libros = I was trying not to stack more books

Notice there is no preposition between intentar and the infinitive. Unlike empezar a, intentar goes straight to the infinitive.

It means not to stack any more books or not to pile more books up.

Breakdown:

• apilar = to stack, pile up
• libros = books
• más = more
• no apilar más libros = not to stack more books

In context, it suggests there were already books on the chair, and the speaker was trying not to add even more.

A natural English translation would often be:

• trying not to pile any more books onto the chair or
• trying not to stack more books on the chair

Because más libros focuses on quantity: more books.

• más libros = additional books, a greater number of books
• otros libros = other books, different books

So:

• no apilar más libros = not to stack any more books
• no apilar otros libros would mean something more like not stacking different/other books, which is a different idea

Here the issue is quantity, not identity, so más is the right choice.

Because sobre means on top of, which fits the idea of stacking books onto a chair.

• sobre la silla = on the chair / on top of the chair
• en la silla = in the chair, or sometimes seated in the chair depending on context

Since books are being piled physically onto the surface of the chair, sobre is a good choice.

You may also hear:

• en la silla in some contexts
• encima de la silla = on top of the chair

But sobre la silla is clear and correct, and it sounds a bit more precise or slightly more literary than en la silla.

Spanish often uses the definite article where English might use the, my, or sometimes no article at all, depending on context.

Here:

• la cama = the bed
• la silla = the chair

They refer to specific, known objects in the situation. Even if English might sometimes say bed or chair more loosely, Spanish commonly uses the article with concrete nouns in a scene like this.

Also, in Spanish, the article is very frequent with everyday objects:

• cerró la puerta = she closed the door
• levantó la mano = he raised his hand

So la cama and la silla sound completely normal.

The word order is quite normal.

Main structure:

• Mi hermana empezó a deshacer la cama
• mientras yo intentaba no apilar más libros sobre la silla

This is basically:

• subject + verb + complement
• while
  • subject + verb + complement

Spanish word order is often flexible, but this sentence uses a straightforward order that matches the flow of the story clearly.

You could move parts around for emphasis, but the original version sounds natural and easy to follow.

Yes, apilar is a real and correct verb, but it is a bit more specific than a more general verb like poner.

• apilar = to stack, pile up neatly or in a heap
• poner = to put
• dejar = to leave
• amontonar = to heap up, pile up messily

So:

• apilar más libros sobre la silla suggests stacking books one on top of another
• poner más libros sobre la silla would just mean putting more books there, without the idea of a stack

So apilar is a good choice if the sentence wants that image of books accumulating in a pile.

Then Spanish would likely use the imperfect for both verbs.

For example:

• Mi hermana deshacía la cama mientras yo intentaba no apilar más libros sobre la silla.

That would mean:

• My sister was unmaking the bed while I was trying not to stack more books on the chair.

The original sentence is different because empezó highlights the moment when the sister started the action. That gives the sentence a more specific narrative event.

So the original version focuses on:

• one action beginning at a certain moment
• another action already in progress at the same time