Mi hermano se siente mejor hoy.

In Spanish, the short possessive adjective goes before the noun:

• mi hermano = my brother
• tu casa = your house
• su amigo = his/her/your/their friend

Mío is a different kind of possessive word. It is usually used after a noun or on its own:

• un hermano mío = a brother of mine
• El libro es mío = The book is mine

So in this sentence, mi hermano is the normal way to say my brother.

Spanish often leaves out subject pronouns when they are not needed. The verb form usually makes the subject clear.

• Mi hermano se siente mejor hoy.
• You do not need él here.

Because mi hermano is already the subject, adding él would be unnecessary. Spanish does this much more often than English.

Here, se siente comes from the verb sentirse, which means to feel in the sense of physical or emotional condition.

So:

• se siente mejor = feels better

This is a very common verb for how someone feels:

• Me siento cansado. = I feel tired.
• Se siente mal. = He/She feels sick/bad.

In your sentence, se is part of the verb sentirse.

Because Spanish often uses sentirse for how someone feels.

Compare:

• sentir = to feel, to sense, to regret
• sentirse = to feel a certain way

Examples:

• Siento frío. = I feel cold / I sense cold
• Me siento enfermo. = I feel sick
• Siento que algo anda mal. = I feel that something is wrong

For personal condition or state, sentirse is usually the natural choice. That is why Mi hermano se siente mejor hoy sounds right for My brother feels better today.

Not exactly. In this sentence, se is the reflexive pronoun that goes with sentirse.

For this verb, the reflexive pronoun changes with the subject:

• me siento = I feel
• te sientes = you feel
• se siente = he/she/you feel
• nos sentimos = we feel
• se sienten = they/you all feel

So se here is not really translated as himself. It is just part of the verb structure sentirse.

Because mejor is the normal comparative form meaning better.

• bien = well
• mejor = better

So:

• se siente bien = he feels well
• se siente mejor = he feels better

Más bien usually does not mean better. It often means something like rather, more like, or pretty much depending on context.

Examples:

• No está enojado; más bien está preocupado. = He isn’t angry; rather, he’s worried.

So for improvement in how someone feels, use mejor.

Yes. Mi hermano está mejor hoy is also very natural and very common.

There is a slight difference in focus:

• se siente mejor = emphasizes how he feels
• está mejor = emphasizes his condition/state

In many everyday situations, both can work and mean almost the same thing.

For example, if someone was sick:

• Mi hermano se siente mejor hoy.
• Mi hermano está mejor hoy.

Both are good Spanish.

Yes, hoy can move around. Spanish word order is often flexible.

All of these are possible:

• Mi hermano se siente mejor hoy.
• Hoy mi hermano se siente mejor.
• Mi hermano hoy se siente mejor.

The version with hoy at the end is very natural and neutral. Putting hoy first can give it a little more emphasis, like Today, my brother feels better.

Usually it means brother when talking about family:

• Mi hermano = my brother

But hermano/hermana can also be used in other contexts, such as:

• religious contexts: brother/sister
• very warm or informal speech in some places: something like brother, bro, or fellow human, depending on context

In this sentence, though, the normal meaning is simply my brother.

By itself, se siente could mean:

• he feels
• she feels
• you feel (formal singular)

The subject tells you which one it is. Here the subject is mi hermano, and hermano is masculine singular, so it means he feels better.

If it were feminine, you might have:

• Mi hermana se siente mejor hoy. = My sister feels better today.

You change both the possessive and the reflexive verb form.

Examples:

• Yo me siento mejor hoy. = I feel better today.
• Tú te sientes mejor hoy. = You feel better today.
• Mi hermano se siente mejor hoy. = My brother feels better today.
• Nosotros nos sentimos mejor hoy. = We feel better today.
• Mis hermanos se sienten mejor hoy. = My brothers feel better today.

This is a good pattern to memorize: me siento, te sientes, se siente, nos sentimos, se sienten.

A helpful approximation is SYEN-teh.

Break it into two syllables:

• sien
• te

A few pronunciation notes:

• ie in siente is a diphthong, so it sounds roughly like ye in one syllable: syen
• the t is a clean Spanish t, lighter than a typical English t
• the final e is pronounced, like eh

So se siente mejor hoy sounds roughly like:

• seh SYEN-teh meh-HOR oy

That is only an approximation, but it is close enough to help beginners.