La meva germana porta una camisa avui.

In Catalan, a possessive like meva is very often used together with the definite article: el, la, els, les.

So:

• la meva germana = my sister
• el meu pare = my father

This is different from English, where you normally say just my sister, not the my sister.

Catalan does sometimes leave out the article in a few special situations, especially in some family terms used very directly, but la meva germana is the normal full form learners should use.

Because germana is a feminine singular noun, and the possessive has to agree with it.

Here is the pattern:

• el meu germà = my brother
• la meva germana = my sister
• els meus germans = my brothers / my siblings
• les meves germanes = my sisters

So meva matches the gender and number of germana.

Yes, portar can mean both to carry and to wear, depending on context.

Examples:

• Porta una bossa. = He/She is carrying a bag.
• Porta una camisa. = He/She is wearing a shirt.

So in your sentence, because the object is una camisa, the meaning is naturally is wearing a shirt, not is carrying a shirt.

Camisa is a feminine singular noun, so the indefinite article must also be feminine singular:

• un for masculine singular
• una for feminine singular

Examples:

• un llibre = a book
• una camisa = a shirt

So una camisa is grammatically correct because camisa is feminine.

Catalan often leaves out subject pronouns when the verb already makes the subject clear. This is similar to Spanish.

• porta already tells you it is he/she/it wears
• so La meva germana porta... is completely natural

You could say Ella porta una camisa avui, but it would usually be for emphasis, contrast, or clarity. In ordinary sentences, Catalan prefers to omit the pronoun.

Yes, avui can move around. Catalan word order is somewhat flexible, especially with time expressions.

All of these are possible:

• La meva germana porta una camisa avui.
• Avui la meva germana porta una camisa.
• La meva germana avui porta una camisa.

The version with avui at the end is perfectly fine. Putting avui at the beginning often gives it a little more emphasis, like Today, my sister is wearing a shirt.

Avui is an adverb of time. It means today.

That is why it does not need a preposition. English also says simply today, not in today.

Other examples:

• Avui plou. = It’s raining today.
• Treballo avui. = I’m working today.

Usually, yes, but it depends a little on context.

In Catalan:

• camisa usually means a shirt, especially a more traditional button-up shirt
• samarreta usually means a T-shirt

So if the sentence says una camisa, many learners should picture a shirt rather than a T-shirt.

Yes. Dur can also mean to wear or to carry, and in some regions or styles you may hear:

• La meva germana duu una camisa avui.

But porta is extremely common and very natural. For most learners, portar is the safest everyday verb to use for wear.

Yes. Porta is the present tense form of portar for he/she/it.

Catalan simple present can often translate into English in more than one way, depending on context:

• She wears a shirt today
• She is wearing a shirt today

In this sentence, English would most naturally say My sister is wearing a shirt today, but Catalan uses the simple present porta.

A simple learner-friendly guide:

• meva ≈ MEH-vuh
• germana ≈ zher-MAH-nuh or jer-MAH-nuh, depending on accent
• porta ≈ POR-tuh
• avui ≈ uh-VUI or ah-VUI

A couple of useful notes:

• The g in germana is often like the s in measure before e
• ui in avui forms a diphthong, so it sounds like one glide, not two fully separate vowels

Pronunciation does vary across Catalan-speaking regions, but these approximations are a good start.