¿Puedes avisar a mi hermana si llego tarde?

Puedes is the tú form of poder, so here it means can you and makes the sentence an informal request.

• ¿Puedes avisar...? = informal, singular
• ¿Puede avisar...? = formal, singular
• ¿Podrías avisar...? = softer / more polite, like could you...

In much of Latin America, whether you use tú or usted depends on the country and the relationship between the speakers.

Here avisar means something like to let someone know, to inform, or to tell.

It is very common in everyday Spanish. In this sentence, avisar a mi hermana means to let my sister know.

It is similar to decir, but avisar often suggests giving someone notice or a heads-up.

That a is needed because the verb is being used with a person: avisar a alguien.

So:

• avisar a mi hermana
• avisar a Juan
• avisar a tus padres

This is related to the personal a, which often appears when the object is a specific person.

You can hear both versions.

The sentence ¿Puedes avisar a mi hermana si llego tarde? is fine as it is. But many speakers, especially in everyday speech, also use a pronoun:

• ¿Le puedes avisar a mi hermana si llego tarde?
• ¿Puedes avisarle a mi hermana si llego tarde?

That extra le refers to mi hermana. Using it is very common, but leaving it out here is also acceptable.

Because si without an accent means if.

• si = if
• sí = yes

So in this sentence, si llego tarde means if I arrive late / if I’m late.

Because Spanish often uses the present tense after si when talking about a real future possibility.

So:

• si llego tarde = if I arrive late / if I’m late

This is normal Spanish grammar. English does something similar:

• If I arrive late, call me.

You do not normally use the future after this kind of si in standard Spanish.

After si meaning if, Spanish normally uses the present indicative, not the future, for real conditions.

So Spanish says:

• si llego tarde

not:

• si llegaré tarde

A simple rule that helps is:

• si + present indicative for a real future possibility

Example:

• Si tengo tiempo, voy.
• Si llego tarde, avísale.

Because this si clause does not take the subjunctive.

When si means if and you are talking about a real or possible condition, Spanish uses the indicative:

• si llego tarde
• si vienes
• si puede

The subjunctive is used in other kinds of clauses, but not in this basic if structure.

In this sentence, si llego tarde is the condition for the request: if I arrive late, can you let my sister know?

The exact content of what you want the person to tell your sister is understood from context.

If you wanted to make the message itself more explicit, you could say something like:

• ¿Puedes avisarle a mi hermana que llego tarde?
• ¿Puedes avisarle a mi hermana que voy a llegar tarde?

So:

• si llego tarde = if I end up being late
• que llego tarde = that I’m late

Yes. You can say:

• Si llego tarde, ¿puedes avisar a mi hermana?

This means the same thing. The difference is mostly emphasis and flow.

• ¿Puedes avisar a mi hermana si llego tarde? = starts with the request
• Si llego tarde, ¿puedes avisar a mi hermana? = starts with the condition

Both are natural.

Yes. Depending on the situation, many speakers might say:

• ¿Puedes avisarle a mi hermana si voy a llegar tarde?
• ¿Puedes avisarle a mi hermana que voy a llegar tarde?

These can sound a little clearer because voy a llegar tarde makes the future idea more explicit, and avisarle includes the pronoun that many speakers naturally use in conversation.

But the original sentence is still correct and understandable.