Mi hermano es tan despistado que deja las llaves en cualquier sitio y luego desconfía de su propia memoria.

It means so ... that ...

So:

• tan despistado = so absent-minded
• que deja las llaves... = that he leaves his keys...

This is a very common Spanish pattern:

• Es tan alto que no cabe por la puerta. = He’s so tall that he can’t fit through the door.
• Habla tan rápido que no lo entiendo. = She speaks so fast that I can’t understand her.

Use tan before adjectives and adverbs:

• tan despistado
• tan rápido

But with nouns, Spanish usually uses tanto/a/os/as ... que:

• Tiene tanta paciencia que nunca se enfada.

Despistado means something like:

• absent-minded
• forgetful
• scatterbrained
• easily distracted / not very aware of where things are

In this sentence, it strongly suggests someone who is always misplacing things or getting confused.

In Spain, despistado is very common in everyday speech.

Examples:

• Soy muy despistado con las fechas. = I’m very forgetful about dates.
• Siempre va despistada. = She’s always a bit out of it / absent-minded.

Because it agrees with mi hermano, which is masculine singular.

Agreement rules:

• hermano → despistado
• hermana → despistada
• hermanos → despistados
• hermanas → despistadas

So:

• Mi hermano es tan despistado...
• Mi hermana es tan despistada...

Because in Spanish, when it is already obvious whose item you mean, Spanish often uses the definite article (el, la, los, las) instead of a possessive.

So:

• deja las llaves literally = leaves the keys
• but naturally it means leaves his keys

This is very common in Spanish.

Examples:

• Me duele la cabeza. = My head hurts.
• Se lavó las manos. = He washed his hands.
• Busca las gafas. = She’s looking for her glasses.

You could say sus llaves, but las llaves sounds more natural here because the owner is already clear from context.

Cualquier sitio means any place or just anywhere.

Here it gives the idea that he leaves the keys in random places, without paying attention.

• en cualquier sitio = anywhere / in any place / just anywhere

In Spain, sitio is a very common everyday word for place.

Similar options:

• en cualquier lugar = a bit more neutral/formal
• en cualquier parte = very common too, often anywhere

So all of these can be close in meaning:

• deja las llaves en cualquier sitio
• deja las llaves en cualquier parte
• deja las llaves en cualquier lugar

Because cualquier is the shortened form of cualquiera used before a singular noun.

So:

• cualquier sitio
• cualquier día
• cualquier cosa

But when it stands alone, you normally use cualquiera:

• Cualquiera puede cometer errores. = Anyone can make mistakes.
• No es un libro cualquiera. = It’s not just any book.

This shortening is very common in Spanish:

• alguno → algún before masculine singular nouns
• bueno → buen before masculine singular nouns
• primero → primer before masculine singular nouns

Because the sentence describes a habit or a general characteristic, not a single event.

Spanish often uses the present tense for repeated actions and general truths.

So here:

• deja las llaves... = he leaves the keys... / he tends to leave the keys...
• desconfía... = he distrusts... / he doesn’t trust...

It implies this is the kind of thing he regularly does.

If you wanted a one-time event, you would normally use a past tense:

• dejó las llaves en cualquier sitio y luego desconfió de su memoria.

Yes. Desconfiar normally takes the preposition de.

• desconfiar de alguien = to distrust someone
• desconfiar de algo = to distrust something
• desconfiar de uno mismo = to distrust oneself

So:

• desconfía de su propia memoria = he distrusts his own memory

More examples:

• Desconfío de esa explicación. = I don’t trust that explanation.
• Ella desconfía de todo el mundo. = She distrusts everyone.

This is something learners usually just have to memorize with the verb.

Su propia memoria means his own memory.

The word propia adds emphasis. It makes the phrase stronger and more explicit:

• su memoria = his memory
• su propia memoria = his own memory

So the sentence suggests: he is so absent-minded that after leaving the keys somewhere, he ends up not even trusting his own recollection.

Examples:

• No confía en su propio juicio. = He doesn’t trust his own judgment.
• Ella escribió su propia versión. = She wrote her own version.

In Spanish, su does not change for the gender of the owner. It changes only for the number of the thing possessed.

So:

• su hermano = his/her/your/their brother
• su memoria = his/her/your/their memory
• sus llaves = his/her/your/their keys

That means su can be ambiguous out of context, but here the context makes it clear that it refers to mi hermano.

Very close, yes. Here luego means then / afterwards.

• deja las llaves... y luego desconfía...
• he leaves the keys... and then distrusts...

In many contexts, luego and después are interchangeable:

• Luego te llamo. = I’ll call you later.
• Después te llamo. = I’ll call you later.

A small note: in some contexts luego can also mean later rather than strictly then, so the exact translation depends on context.

Not with exactly the same meaning.

• dejar las llaves en cualquier sitio = to leave the keys somewhere / put them down somewhere
• olvidar las llaves = to forget the keys

These are related ideas, but they are not identical.

In this sentence, the point is that he puts or leaves the keys in random places. That is why deja is the right verb.

If you said:

• olvida las llaves that would usually mean he forgets to take them, or leaves them behind entirely.

It is pronounced roughly like des-con-FEE-a, with the stress on -fí-.

The accent mark shows where the stress goes and also helps show that í-a is pronounced in separate syllables.

So:

• desconfía = des-con-fí-a

Without going too deeply into spelling rules, the written accent tells you that this form is stressed differently from what you might otherwise expect.

This is common in verb forms like:

• confía
• envía
• sonríe

Yes, sitio is very natural in Spain and very common in everyday speech.

That said, the whole sentence is understandable across the Spanish-speaking world. Nothing in it is uniquely Spanish in the sense of being hard to understand elsewhere.

A few regional notes:

• In Spain, sitio is extremely common for place
• In Latin America, lugar or parte may also be very common depending on the country

So en cualquier sitio sounds very natural in Spain Spanish, which fits your target variety well.