Mi hermano se lava la cara antes de dormir.

Because this is a reflexive construction: the subject is doing the action to himself.

• lavar = to wash
• lavarse = to wash oneself

So:

• Mi hermano lava el coche = My brother washes the car
• Mi hermano se lava = My brother washes himself

Here, se is the reflexive pronoun for él/ella/usted.

With body parts in Spanish, it is very common to use the definite article (la, el, los, las) instead of a possessive like su when the owner is already clear.

So Spanish prefers:

• se lava la cara

rather than:

• se lava su cara

The reflexive pronoun se already tells you whose face it is: his own.

Using su cara is not impossible, but it sounds less natural here unless you want to add emphasis or contrast.

Lava is the 3rd person singular present indicative of lavar.

So it matches mi hermano:

• yo lavo
• tú lavas
• él/ella lava

Because the sentence is reflexive, you get:

• él se lava

It can technically do either, depending on context, but in this sentence it most naturally sounds like a habit or routine.

So:

• Mi hermano se lava la cara antes de dormir
  = My brother washes his face before sleeping / before bed

It sounds like something he usually does, not necessarily something happening at this exact moment.

Because antes needs de before a noun or an infinitive.

Examples:

• antes de dormir
• antes de comer
• antes de la cena

So antes dormir is not correct standard Spanish.

A useful pattern to remember is:

• antes de + infinitive = before doing something

Both are possible in some contexts, but they are not exactly the same.

• dormir = to sleep
• dormirse = to fall asleep / to go to sleep

So:

• antes de dormir = before sleeping / before going to sleep
• antes de dormirse = before falling asleep

In your sentence, antes de dormir is very natural and broad. It refers to the time before sleep.

In Spanish, reflexive pronouns usually go before a conjugated verb.

So:

• se lava

not:

• lava se

However, with an infinitive, gerund, or affirmative command, the pronoun is attached to the end:

• lavarse
• lavándose
• lávate

So in your sentence, since lava is a conjugated verb, se goes before it.

Yes. Spanish often omits the subject when it is understood from context.

So you could say:

• Se lava la cara antes de dormir

and it could mean:

• He washes his face before sleeping

But adding mi hermano makes it explicit who you are talking about.

It is grammatically possible, but it is not the most natural way to say it.

For actions people do to their own body, Spanish normally prefers the reflexive pattern:

• Mi hermano se lava la cara

Using su cara can sound overly explicit, or as if you are stressing his face in contrast with someone else’s.

So learners should usually choose:

• se lava la cara

No. In standard Spanish, h is silent.

So hermano is pronounced roughly like ermano.

Also, in cara, the c before a is a hard k sound:

• cara ≈ KAH-ra

That pronunciation is the same in Spain.