Soror ad dextram vertit, sed frater ad sinistram ambulat.

They are nominative singular because they are the subjects of the two clauses:

• soror = the one who turns
• frater = the one who walks

In Latin, the nominative case is typically used for the subject of a verb.

Because ad commonly means to or toward, and it takes the accusative case. Here it shows direction:

• ad dextram = to the right
• ad sinistram = to the left

This is a very common Latin pattern for movement or orientation toward a direction.

They are adjectives used in a directional expression with an understood feminine noun, often explained as something like partem (side) or sometimes manum (hand). So Latin says, in effect, to the right side and to the left side, but the noun is left out.

That is why the forms are feminine singular accusative:

• dextram
• sinistram

Both are third-person singular verb forms, matching the singular subjects soror and frater.

• vertit = turns
• ambulat = walks

In this sentence they are understood as present indicative active: the sister turns, and the brother walks.

Sed means but. It connects the two clauses and shows a contrast:

• the sister turns one way,
• but the brother walks the other way.

So sed is a coordinating conjunction that introduces an opposition or contrast.

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

This sentence has a very natural and clear order:

• Soror ad dextram vertit
• sed frater ad sinistram ambulat

But Latin could rearrange the words for emphasis without changing the basic meaning. The endings, not just the position, tell you what each word is doing.

Yes, but they belong to the third declension, which can look less regular to English-speaking beginners than first- or second-declension nouns.

Their dictionary forms are:

• soror, sororis = sister
• frater, fratris = brother

So even though they do not end in -a or -us, they are perfectly normal Latin nouns with their own declension pattern.