Mi hermano se pasa el día bromeando, y a veces nadie sabe si habla en serio.

In this sentence, pasarse + time expression + gerund is a very common way to say to spend time doing something, often with a slightly emphatic or slightly critical tone.

• Mi hermano pasa el día... = he spends the day...
• Mi hermano se pasa el día... = he spends the whole day..., he is always...

The se here is part of the pronominal verb pasarse, not a literal reflexive himself. It often makes the expression feel more idiomatic and more expressive.

So se pasa el día bromeando suggests something like:

• he spends all day joking around
• he is always joking

This is a very common Spanish structure:

pasarse + a period of time + gerund

Examples:

• Se pasa la mañana estudiando = She spends the morning studying.
• Nos pasamos la noche hablando = We spent the night talking.
• Se pasan el verano viajando = They spend the summer travelling.

So in your sentence:

• se pasa = spends
• el día = the day / all day
• bromeando = joking

Together: he spends the day joking / he spends all day joking around.

Because after pasarse + time expression, Spanish normally uses a gerund to show the activity occupying that time.

So:

• se pasa el día bromeando = he spends the day joking

Not:

• se pasa el día bromear ❌

This is similar to English:

• He spends the day joking not
• He spends the day joke ❌

The Spanish gerund here works very naturally after expressions of time spent doing something.

In this expression, el día often has the sense of the whole day or all day.

So:

• Se pasa el día bromeando does not usually mean one specific calendar day in a neutral way.
• It usually suggests a habitual idea: he spends all day joking around.

This is an idiomatic use. Spanish often uses:

• el día
• la mañana
• la tarde
• la noche

in this pattern to mean that someone spends that whole stretch of time doing something.

Yes, but the structure would usually change a little.

Natural options include:

• Mi hermano está todo el día bromeando
• Mi hermano se pasa todo el día bromeando

Both are natural.

Compare:

• se pasa el día bromeando = idiomatic and very common
• se pasa todo el día bromeando = even more explicit: he spends the whole day joking
• está todo el día bromeando = also very common, similar idea

So el día already gives that all day feeling, and todo el día just makes it even more explicit.

They are very close.

• bromear = to joke, to kid around
• hacer bromas = to make jokes

In many contexts they are interchangeable:

• Mi hermano se pasa el día bromeando
• Mi hermano se pasa el día haciendo bromas

But bromear often feels a bit more general and flowing as an activity, while hacer bromas can focus more on the individual jokes.

Here si means whether / if in an indirect question.

• nadie sabe si habla en serio = nobody knows whether he is speaking seriously

This is not the conditional if of if it rains, we stay home. It is the if/whether used after verbs like:

• saber = to know
• preguntar = to ask
• ver = to see
• decidir = to decide

Examples:

• No sé si viene = I don’t know if / whether he is coming.
• Pregúntale si quiere café = Ask him if he wants coffee.

Because after saber si, Spanish normally uses the indicative when talking about a real uncertainty about a factual situation.

• nadie sabe si habla en serio = nobody knows whether he is serious / whether he means it

The speaker is not presenting hablar en serio as something hypothetical or desired; they are simply saying that people do not know which of two real possibilities is true:

• either he is joking
• or he is speaking seriously

That is why habla in the indicative is the normal choice.

Hablar en serio is a fixed expression meaning:

• to be serious
• to speak seriously
• to mean what one says

So:

• No sé si habla en serio = I don’t know if he’s serious
• ¿Hablas en serio? = Are you serious?
• Te lo digo en serio = I’m telling you seriously / I really mean it

Even though the literal words are speak in serious, the natural English meaning is often simply be serious or mean it.

Because nadie is the normal word for nobody / no one when referring to people in general.

• Nadie sabe... = Nobody knows...

Ninguno usually means none / not one, and it normally refers back to a group already mentioned or understood.

Compare:

• Nadie sabe si habla en serio = Nobody knows whether he’s serious.
• De mis amigos, ninguno sabe francés = Of my friends, none knows French.

So here nadie is the natural choice.

Because the clause si habla en serio already functions as the direct object of sabe.

In other words, what nobody knows is:

• si habla en serio

So Spanish does not need an extra object pronoun here.

Compare:

• Nadie lo sabe = Nobody knows it.
• Nadie sabe si habla en serio = Nobody knows whether he’s serious.

Using both together would normally be unnecessary in this sentence.

Not necessarily. In this sentence, the present tense has a habitual meaning.

• Mi hermano se pasa el día bromeando = my brother spends all day joking around / is always joking
• a veces nadie sabe = sometimes nobody knows
• si habla en serio = whether he is serious

So the sentence describes a usual pattern, not just one moment happening right now.

Spanish present tense often does this, just like English:

• He jokes all day
• Sometimes nobody knows if he’s serious

Yes, but that would change the meaning.

• está bromeando = he is joking right now
• está hablando en serio = he is speaking seriously right now

Your original sentence is more general and habitual:

• se pasa el día bromeando = he spends all day joking
• a veces nadie sabe si habla en serio = sometimes nobody knows if he is serious

So:

• está + gerund focuses on what is happening at the moment
• the original sentence describes a repeated characteristic of your brother

Yes, very natural.

All of these sound very normal in Spain:

• se pasa el día bromeando
• nadie sabe si habla en serio
• hablar en serio

The tone is conversational and idiomatic. A Spanish speaker from Spain would easily say something like this in everyday speech.