Mi sobrina se quedó mirando una hormiga que llevaba una hoja casi tan grande como ella.

Quedarse + gerund is a very common Spanish structure meaning to remain/stay doing something or to end up doing something for a while.

So se quedó mirando means something like:

• she stood there looking
• she kept looking
• she was left staring

If you said miró una hormiga, that would simply mean she looked at an ant.
But se quedó mirando adds the idea that she paused and stayed focused on it.

Here, quedarse means to stay, to remain, or sometimes to be left in a certain state.

The se is part of the verb quedarse. Compare:

• quedar = to remain, to be located, to arrange
• quedarse = to stay, to remain behind, to end up in a state

So in this sentence:

• se quedó mirando = she stayed looking / she remained staring

The se is not optional here if you want this meaning.

Quedó is preterite because it presents the action as a completed event: she ended up stopping and staring at that moment.

Spanish often uses the preterite for the moment when someone entered a state:

• se quedó callado = he went quiet / he fell silent
• se quedó dormido = he fell asleep
• se quedó mirando = she stopped and stared / she remained looking

So the preterite marks the change into that state.

After quedarse, Spanish very often uses a gerund to describe the activity that continues:

• se quedó hablando = he stayed talking
• se quedó pensando = she stayed thinking
• se quedó mirando = she stayed looking

Using a + infinitive after quedarse is not the normal pattern here.

So se quedó mirando is the natural way to say that she remained engaged in that action.

Yes — llevar often means to carry or to wear. Here it means was carrying.

So:

• una hormiga que llevaba una hoja = an ant that was carrying a leaf

Spanish uses llevar very naturally for transporting or holding something along while moving.

Llevaba is the imperfect, which is used here to describe an action that was in progress in the background.

The sentence sets up a scene:

• the niece stopped and stared
• at an ant
• that was carrying a leaf

So:

• se quedó = main event, completed moment
• llevaba = ongoing background action

If you used llevó, it would sound more like a finished, single action, which is less natural in this descriptive context.

It refers to una hormiga.

So:

• una hormiga que llevaba una hoja = an ant that was carrying a leaf

The word que here means that / which / who, introducing a relative clause that describes the ant.

Yes, hoja can mean several things depending on context:

• leaf of a plant
• sheet of paper
• blade in some contexts

Here, because it is being carried by an ant, the natural meaning is leaf.

This is the standard Spanish pattern for comparisons of equality:

• tan + adjective + como = as + adjective + as

So:

• tan grande como ella = as big as her
• casi tan grande como ella = almost as big as her

Examples:

• tan alto como yo = as tall as me
• tan rápido como tú = as fast as you

In this sentence, ella most naturally refers to la hormiga, the ant.

So the meaning is:

• the ant was carrying a leaf almost as big as the ant itself

Even though ella could grammatically refer to another feminine noun, the context strongly points to hormiga.

Because ella is enough here to refer back naturally to the ant.

Spanish often uses a simple pronoun in comparisons:

• tan alto como él
• tan grande como ella

Using sí misma would sound more emphatic and less natural in this sentence.

So casi tan grande como ella is the normal way to say almost as big as it/her.

Grande is the normal full form of the adjective.

Gran is a shortened form used before a singular noun, and it often adds a more figurative meaning like great or important:

• una gran idea = a great idea
• una casa grande = a big house

Here, grande comes after tan and is part of a comparison, so it must be grande:

• tan grande como ella

Normally, if mirar directly takes a person or animal as its object, a is often possible or expected, especially with specific beings:

• miró a su madre
• miraba al perro

But after quedarse mirando, Spanish frequently leaves the object without a, especially in more descriptive narration:

• se quedó mirando una hormiga

This is natural and idiomatic. You may also hear se quedó mirando a una hormiga, but the version without a sounds very normal in this context.

Sobrina simply means niece.

• mi sobrina = my niece

There is no special nuance here beyond the family relationship.

Yes, that is a very natural English rendering.

A few possible translations are:

• My niece stood staring at an ant carrying a leaf almost as big as she was.
• My niece stopped to stare at an ant that was carrying a leaf almost as big as itself.
• My niece kept looking at an ant carrying a leaf almost as big as the ant.

They all capture the basic meaning, though se quedó mirando especially suggests that she remained absorbed in watching it.