El siguiente es mi hermano; está en la fila.

El siguiente is an adjective being used like a noun, meaning the next one / the next person. Spanish often lets you omit a noun when it’s obvious from context (like persona, chico, hermano, en la lista, etc.). So El siguiente es mi hermano is essentially (La) persona siguiente es mi hermano, but with the noun understood.

It’s masculine because it refers to a male person here (mi hermano). If you were introducing a sister, you’d say La siguiente es mi hermana.
In general:

• male / mixed / unknown → El siguiente
• female → La siguiente

Sometimes, but El siguiente is the most natural for the next one in order (like the next person to be called). El próximo can work, but it can feel a bit more like the upcoming one. In many real contexts (a line, a list, calling turns), El siguiente is the default.

Because being in a place/position is temporary/location-based, so Spanish uses estar: estar en + place.

• Está en la fila = he is (located) in the line.
  Using es would sound wrong here because ser is for identity/characteristics, not location.

The semicolon links two closely related independent clauses:

• El siguiente es mi hermano
• está en la fila

You could also write:

• El siguiente es mi hermano. Está en la fila. (most common)
• El siguiente es mi hermano, está en la fila. (possible, but the comma splice is often avoided in careful writing)

Not usually. Spanish commonly drops subject pronouns when the verb already shows who it is. Está already implies he/she/you(formal), and the context makes it clear it’s your brother.
You might add él for emphasis or contrast (e.g., Él está en la fila, no yo).

Both, depending on context. In Latin America, la fila is very commonly a queue / a line of people waiting. It can also mean row (like a row of seats), but if someone is waiting their turn, la fila is the normal word.

In Spain, people often say la cola for a queue, though fila is still understood (and used in some contexts). In Latin America, fila is the standard everyday choice for “line/queue.”

The accent distinguishes está (verb form: he/she/it is) from esta (demonstrative: this feminine, as in esta casa = this house).
So:

• está = “is” (from estar)
• esta = “this” (feminine)

Yes, that’s grammatical and natural. The two versions just emphasize different information:

• El siguiente es mi hermano emphasizes who the next person is (good for introductions).
• Mi hermano es el siguiente emphasizes your brother and states he’s next.

It can be used for both people and things. Examples:

• El siguiente es mi hermano (next person)
• La siguiente es la pregunta número dos (next item)
• El siguiente es el capítulo 3 (next section)

It just has to match gender/number: el/la/los/las siguiente(s).

Yes:

• Los siguientes son mis hermanos; están en la fila. (male/mixed group)
• Las siguientes son mis hermanas; están en la fila. (female group)

Notice both the article/adjective (los/las siguientes) and the verb (son / están) change to plural.