Mi hermana cumplirá veintiún años en mayo, y ya tiene pensado invitar a toda la familia.

Cumplirá is the simple future of cumplir. In this sentence, cumplir veintiún años means to turn twenty-one.

So:

• Mi hermana cumplirá veintiún años en mayo = My sister will turn twenty-one in May

In Spanish, cumplir + number + años is the normal way to talk about reaching a certain age.

You could also hear va a cumplir veintiún años, which is the going to future. Both are correct, but cumplirá sounds a bit more neutral and straightforward in written Spanish.

Literally, cumplir means something like to complete or to fulfill, but with age it means to turn a certain age.

Common patterns:

• cumplir años = to have a birthday / to turn a year older
• cumplir veinte años = to turn twenty
• ¿Cuándo cumples años? = When is your birthday?

So in this sentence, cumplirá veintiún años does not mean she is completing twenty-one years in some abstract sense; it is simply the normal Spanish way to say she will turn twenty-one.

Because veintiuno changes to veintiún before a masculine singular noun.

This shortening is called apocopation.

Examples:

• veintiún años
• veintiún libros
• treinta y un días

But:

• veintiuna personas (feminine noun)
• veintiuno when the number stands alone

Even though años is plural in meaning, the rule is based on the noun phrase beginning with the masculine noun año, so Spanish uses veintiún años.

Because age is counted in years, and anything above one uses the plural:

• un año
• dos años
• veintiún años

So veintiún años is exactly the same idea as twenty-one years in English, even though English usually says twenty-one years old or simply turn twenty-one.

In Spanish, months are normally used with en when talking about when something happens:

• en mayo = in May
• en julio = in July
• en 2026 = in 2026

So cumplirá veintiún años en mayo means the birthday will happen in May.

Here ya means something like already, by now, or as it is.

In this sentence:

• ya tiene pensado invitar a toda la familia

the idea is that she has already thought about it or already has the plan in mind.

It gives the sense that the planning is not just a future possibility; she has already decided or already started thinking seriously about inviting everyone.

Tener pensado + infinitive is a very common structure meaning:

• to be planning to...
• to intend to...
• to have it in mind to...

So:

• tiene pensado invitar = she is planning to invite / she intends to invite

Literally, it looks like she has thought-out inviting, but in natural English it is better translated as she plans to invite.

Other examples:

• Tengo pensado viajar este verano = I’m planning to travel this summer.
• Tenemos pensado mudarnos = We’re planning to move.

Because the verb invitar normally takes a before the person or people invited:

• invitar a alguien = to invite someone

So:

• invitar a toda la familia = to invite the whole family

This is just the normal construction of the verb. Since the people being invited are persons, the a is required.

Compare:

• Invitó a sus amigos
• Voy a invitar a mi prima
• Quiere invitar a toda la familia

Because in Spanish, when you mean the whole family, you usually say toda la familia.

• toda la familia = the whole family
• todo el día = the whole day
• toda la semana = the whole week

Without the article, toda familia would sound much less natural here and would suggest a more general meaning, like every family or any family, depending on context.

So a toda la familia specifically means the entire family.

Because Spanish often leaves out possessives when ownership or relationship is already obvious from context.

Here, Mi hermana has already been mentioned, so a toda la familia naturally means the whole family connected to the situation, usually understood as our/the family.

Spanish often prefers the article instead of a possessive when the meaning is clear.

Both are possible, but the nuance is slightly different:

• a toda la familia = the whole family
• a toda su familia = her whole family

The version in the sentence sounds very natural and less heavy.

Because tener pensado + infinitive is a set expression meaning to have planned/intended to do something.

• ya tiene pensado invitar... = she already plans/intends to invite...

If you said ya ha pensado invitar..., it would sound more like she has already thought about inviting..., focusing on the act of thinking rather than on having a settled plan.

So:

• tener pensado + infinitive = having a plan/intention
• haber pensado en + infinitive / haber pensado + noun clause = having thought about something

In this sentence, the idea is that she already has a plan, so tiene pensado fits very well.