Si l'oncle ho descobrís abans, la meva neboda i el meu nebot es posarien tristos.

Because el becomes l' before a vowel sound. This is called elision.

• el + oncle → l'oncle

So l'oncle simply means the uncle.

In Catalan, it is very common to use the definite article + possessive + noun together:

• la meva neboda
• el meu nebot

That is different from English, where we normally just say my niece and my nephew without the.

So this is a normal Catalan structure, especially in standard usage.

Ho is a neutral direct object pronoun. It usually refers to it, but specifically to:

• a fact
• a situation
• something previously mentioned
• an idea or whole clause

So in this sentence, ho descobrís means discovered it / found that out.

Unlike nouns such as el llibre or la carta, ho does not show masculine or feminine gender.

Descobrís is the imperfect subjunctive form of descobrir.

Here it appears because the sentence is a hypothetical conditional:

• Si ... descobrís ...
• ... es posarien ...

This is the standard pattern for an unreal or unlikely condition:

• si + imperfect subjunctive
• conditional in the main clause

So:

• Si l'oncle ho descobrís abans, ...
• ..., es posarien tristos.

This is similar to English patterns like If the uncle found it out earlier, they would become sad.

Because after si in this kind of conditional sentence, Catalan does not normally use the conditional.

The standard pattern is:

• Si
  • imperfect subjunctive
• main clause in the conditional

So:

• correct: Si l'oncle ho descobrís abans, ... es posarien tristos.
• not standard here: Si l'oncle ho descobriria...

This is a very important pattern to learn in Catalan.

In Catalan, object pronouns usually go before a finite verb.

So you get:

• ho descobrís
• el veu
• la coneix
• ens ajudaria

You typically attach pronouns after the verb only with:

• infinitives
• gerunds
• affirmative imperatives

So here, because descobrís is a finite verb form, ho goes before it.

Literally, it is something like would put themselves sad, but that is not how it works idiomatically.

The expression posar-se + adjective means to become or to get + adjective.

Examples:

• posar-se trist = to become sad
• posar-se nerviós = to get nervous
• posar-se content = to become happy

So es posarien tristos means they would become sad.

Because the verb here is posar-se, a pronominal verb.

In this expression, the pronoun is part of the verb:

• posar-se trist
• posar-se malalt
• posar-se vermell

With a third-person plural subject, the pronoun is es:

• ells/elles es posarien

So es is not optional here if you want this meaning of become/get.

Because the subject is plural:

• la meva neboda i el meu nebot

Two people joined by i make a plural subject, so the verb must also be plural:

• singular: la meva neboda es posaria trista
• plural: la meva neboda i el meu nebot es posarien tristos

Because the subject is a mixed-gender plural:

• neboda = feminine
• nebot = masculine

In Catalan, when a plural group includes at least one masculine noun, the adjective normally goes in the masculine plural:

• tristos

So:

• la meva neboda → trista
• el meu nebot → trist
• la meva neboda i el meu nebot → tristos

Here abans means earlier or beforehand.

It is an adverb, and its position is fairly flexible. Putting it at the end is completely natural:

• Si l'oncle ho descobrís abans...

It tells you when the discovering would happen.

If abans is followed by a noun or clause, Catalan often uses d':

• abans d'ahir = the day before yesterday
• abans de sortir = before leaving

But here it stands alone, so just abans is correct.

Normally, no. In this sentence, each noun needs its own determiner structure:

• la meva neboda i el meu nebot

That is especially important here because the nouns have different genders:

• la meva
• el meu

You could rephrase the idea in other ways, but la meva neboda i nebot would sound incomplete or unnatural in standard Catalan.