A mi tía no le gustan los mejillones, pero mi prima los come sin problema.

Yes, they both point to my aunt, but this is very normal in Spanish. It’s called indirect object doubling.

• a mi tía identifies or emphasizes who we are talking about.
• le is the indirect object pronoun that the verb structure requires.

With gustar, this doubling is especially common:

• A mi tía le gustan los mejillones.
• A Juan le encanta el café.

In natural Spanish, leaving out le here would sound wrong.

Because the thing doing the grammatical liking is los mejillones, and that noun is plural.

With gustar, Spanish works differently from English:

• A mi tía le gustan los mejillones
  literally: Mussels are pleasing to my aunt

So the verb agrees with los mejillones, not with mi tía.

Compare:

• Le gusta el mejillón = She likes mussels / the mussel
• Le gustan los mejillones = She likes mussels

Because gustar does not work like English to like.

In English:

• My aunt likes mussels

In Spanish:

• Mussels please my aunt

So:

• los mejillones = grammatical subject
• mi tía = indirect object

That’s why you get:

• A mi tía le gustan los mejillones

and not a verb form agreeing with mi tía.

They are doing two different jobs.

1. In los mejillones, los is the definite article: the
2. In los come, los is a direct object pronoun: them

So:

• los mejillones = the mussels
• los come = she eats them

The second los refers back to mejillones.

Because in Spanish, object pronouns usually go before a conjugated verb.

So:

• Mi prima los come = My cousin eats them

This is the normal placement with a simple conjugated verb.

If the verb structure changes, pronouns can move:

• Mi prima va a comerlos
• Mi prima los va a comer

Both are possible there, but with just come, it must be before:

• los come

Because le is an indirect object pronoun, and la is usually a direct object pronoun.

Here, my aunt is the indirect object with gustar, so Spanish uses:

• le = to him / to her / to you (formal)

So:

• A mi tía le gustan...

not:

• A mi tía la gustan... ❌

By contrast, with a direct object verb:

• La veo = I see her

There, la is correct because her is the direct object.

Spanish often uses the definite article when talking about things in a general sense, especially with verbs like gustar.

So:

• No le gustan los mejillones = She doesn’t like mussels

Even though English often drops the article in general statements, Spanish frequently keeps it.

Other examples:

• Me gusta el café = I like coffee
• No me gustan las arañas = I don’t like spiders

Sin problema is a very common expression meaning:

• without any problem
• no problem
• with no difficulty

Spanish often omits the article in expressions like this after sin.

So:

• los come sin problema = she eats them without any problem / she has no problem eating them

You could also hear:

• sin ningún problema = without any problem at all

That is a bit more explicit, but sin problema is very natural.

Because Spanish usually does not need subject pronouns when the subject is clear.

The verb form and the noun already tell us who is doing the action:

• mi prima los come

Adding ella is possible, but it would usually add emphasis or contrast:

• pero mi prima sí los come
• pero ella los come sin problema

In most neutral sentences, Spanish simply leaves ella out.

Pero means but, and it introduces a contrast between the two parts of the sentence:

• A mi tía no le gustan los mejillones
• pero mi prima los come sin problema

So the idea is:

• my aunt doesn’t like them,
• but my cousin eats them just fine.

It connects two different attitudes toward the same food.

Yes, Spanish word order is more flexible than English, especially with elements like a mi tía.

For example, these are all possible:

• A mi tía no le gustan los mejillones
• Los mejillones no le gustan a mi tía
• No le gustan los mejillones a mi tía

They all mean roughly the same thing, though the focus or emphasis changes slightly.

The version in your sentence is very natural because it introduces the person first:

• A mi tía...

Yes. Mejillones is the standard word for mussels in Spain and across the Spanish-speaking world.

It’s a normal everyday food word, especially common in Spain because mussels are widely eaten there.

Singular and plural:

• el mejillón = the mussel
• los mejillones = the mussels

Because the sentence is contrasting two different ideas:

• liking something
• eating something

These are related, but not identical.

So:

• A mi tía no le gustan los mejillones = she doesn’t like mussels
• mi prima los come sin problema = my cousin eats them with no problem

The second clause does not necessarily mean my cousin loves mussels. It only says she eats them easily or willingly.

No. Prima is specifically female cousin.

• mi primo = my male cousin
• mi prima = my female cousin

So the sentence clearly refers to a female cousin.

In Spanish, no normally goes directly before the verb or before the pronoun + verb group.

So:

• no le gustan

This is the standard way to make the clause negative:

• A mi tía no le gustan los mejillones

You should not place no later in the sentence in this structure.