Questions & Answers about ¿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.
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.