Gemellus frater puellae ridet, sed gemella soror eius adhuc ante speculum stat.

They are the same adjective in different genders.

• gemellus = masculine singular
• gemella = feminine singular

Latin adjectives must agree with the nouns they describe in gender, number, and case:

• gemellus frater = twin brother
• gemella soror = twin sister

So the ending changes because frater is masculine and soror is feminine.

Here puellae is genitive singular, meaning of the girl.

So:

• frater puellae = the girl’s brother

A native English speaker may notice that puellae can also have other meanings in other contexts, such as to/for the girl or even girls. But here the sense is clearly genitive because it shows possession: the brother belongs to, or is associated with, the girl.

You know from context and from the way the words fit together.

In frater puellae, the most natural meaning is the girl’s brother. Latin often uses the genitive after family nouns like frater to show whose brother, sister, father, and so on someone is.

If it meant to the girl, that would be a dative idea, but brother to the girl does not make good sense here. So context points strongly to the genitive: of the girl.

eius means his, her, or its, depending on context. It is the genitive singular form of the pronoun is, ea, id.

In this sentence, eius means her:

• soror eius = her sister

It refers back to puella. So the second clause is talking about the same girl already mentioned earlier.

A useful point: eius does not change form for masculine, feminine, or neuter. Context tells you whether it means his, her, or its.

This is a very common Latin question.

Latin uses:

• suus, sua, suum for a reflexive possessor, meaning the possessor is the subject of the same clause
• eius for someone else’s possession

In the second clause, the subject is gemella soror. But the owner is the girl mentioned earlier, not the sister herself. So Latin uses eius.

Compare:

• soror eius stat = her sister is standing
  (someone else’s sister, here the girl’s)
• soror sua stat would suggest something like her own sister is standing, where the possession reflects back to the subject in a way Latin handles differently and more carefully than English

So eius is the correct non-reflexive form here.

Latin shows relationships mainly through endings, not just word order.

Here is how the parts group together:

• gemellus frater: both are masculine singular nominative, so they go together
• puellae: genitive singular, depending on frater
• ridet: the verb for that first subject

Then:

• gemella soror: both are feminine singular nominative, so they go together
• eius: shows possession, her
• adhuc: adverb, still
• ante speculum: prepositional phrase, in front of the mirror
• stat: the verb for the second subject

So the endings help you see the structure even if the order is not exactly like English.

Classical Latin has no articles. That means there is no direct equivalent of English the or a/an.

So:

• frater can mean brother, a brother, or the brother
• soror can mean sister, a sister, or the sister

English has to choose an article when translating, but Latin usually leaves that to context.

Speculum is accusative singular because it follows the preposition ante.

Here ante means before or in front of, and with this meaning it takes the accusative:

• ante speculum = in front of the mirror

So even though English uses of the mirror or the mirror without thinking much about case, Latin marks the object of the preposition with the accusative ending.

Adhuc means still, up to now, or yet, depending on context.

In this sentence, it means still:

• adhuc ante speculum stat = is still standing in front of the mirror

It adds the idea that the action is continuing. The sister has not moved away from the mirror yet.

Latin word order is much more flexible than English word order because Latin uses endings to show grammatical roles.

English depends heavily on order:

• The girl’s brother laughs

Latin can arrange words more freely:

• Gemellus frater puellae ridet

That order is perfectly normal. Latin writers often move words around for emphasis, rhythm, or style. Even so, there are common patterns, and this sentence is not especially unusual.

So the most important thing is not to read Latin by strict English order, but to watch the endings and group the words by their forms.

Both verbs are third person singular present tense.

• ridet = he laughs, she laughs, or it laughs
• stat = he stands, she stands, or it stands

Latin usually does not need a separate subject pronoun like he or she, because the verb ending already tells you the person and number. The actual subject is supplied by the noun in the sentence:

• frater ridet = the brother laughs
• soror stat = the sister stands

Formally, puellae could be:

• genitive singular: of the girl
• dative singular: to/for the girl
• nominative plural: girls

But in this sentence, plural does not fit. If puellae were nominative plural, it would not make good sense with frater. The phrase frater puellae is most naturally understood as the girl’s brother.

So this is a good example of how Latin forms can be ambiguous by themselves, but the sentence around them makes the meaning clear.

Yes. Latin often uses an adjective plus a noun where English uses a fixed expression.

So:

• gemellus frater = twin brother
• gemella soror = twin sister

English speakers may feel that twin brother and twin sister are almost like set phrases. Latin expresses the same idea with the ordinary adjective gemellus, gemella agreeing with the noun.