Mi hermana se pasa el día leyendo novelas en español.

It is a very common Spanish pattern: pasarse el día + gerund.

So Mi hermana se pasa el día leyendo novelas en español means My sister spends the whole day reading novels in Spanish or My sister spends all day reading novels in Spanish.

You should learn se pasa el día leyendo as a chunk, not word by word.

Here, se is part of the verb pasarse, which is often used in this expression.

It is not really reflexive in the English sense of herself. Spanish just uses the pronominal form in this idiomatic structure:

• pasarse el día estudiando
• pasarse la tarde hablando
• pasarse la noche trabajando

This structure often sounds very natural and colloquial in Spanish.

Because after pasarse el día, Spanish normally uses a gerund to show the activity:

• se pasa el día leyendo
• se pasa el día estudiando
• se pasa el día durmiendo

So leer would not sound right here. Spanish wants leyendo because the meaning is spends the day doing something.

Because the simple present in Spanish often describes a usual habit or a typical behavior.

So Mi hermana se pasa el día leyendo novelas en español can mean something like:

• My sister spends all day reading novels in Spanish
• My sister is always spending the day reading novels in Spanish

You do not need está pasando here. In fact, está pasando would sound strange in this sentence.

It can be neutral, but very often it carries a sense of emphasis, and sometimes a slight feeling of criticism or exaggeration.

Depending on context, it could mean:

• a neutral fact: She spends all day reading
• mild annoyance: She’s always reading all day
• admiration: She really spends her whole day reading

So the exact tone depends on who says it and how they say it.

Because pasarse el día + gerund is a fixed, common expression.

Here el día already gives the idea of the whole day. So:

• se pasa el día leyendo
• se pasa todo el día leyendo

Both are possible, and both mean roughly the same thing.
Adding todo can make the idea even stronger, but it is not necessary.

It means novels written in the Spanish language.

So en español describes the language of the novels, not where they are from.

That means the novels could be from:

• Spain
• Mexico
• Argentina
• Colombia
• anywhere else, as long as they are in Spanish

This is an important difference:

• novelas en español = novels in the Spanish language
• novelas españolas = Spanish novels, usually meaning novels from Spain

So they are not the same.

For example:

• A novel by a Mexican author is a novela en español
• but it is not necessarily a novela española

When Spanish names a language after en, it normally does not use an article:

• en español
• en inglés
• en francés

Also, names of languages in Spanish are normally written in lowercase, not with a capital letter.

So Spanish writes:

• español
• inglés
• alemán

not Español, Inglés, or Alemán unless the word starts a sentence.

Yes, you can, and it is understandable.

But se pasa el día leyendo is especially common and often sounds more natural for this idea. It can also feel a bit more emphatic, like she spends the whole day doing that.

So:

• Mi hermana pasa el día leyendo... = correct
• Mi hermana se pasa el día leyendo... = very common, often a bit more expressive

Yes. This is a very useful pattern.

You can say:

• Se pasa la mañana estudiando. = He/She spends the morning studying.
• Se pasó la tarde viendo la tele. = He/She spent the afternoon watching TV.
• Nos pasamos la noche hablando. = We spent the night talking.

So the pattern is:

pasarse + a time expression + gerund

That makes this sentence a great model to reuse.