Ma fiancée veut annoncer nos fiançailles à son beau-père et à sa belle-mère pendant le dîner.

Because fiancée is a feminine singular noun: it means female fiancé / the woman I’m engaged to.

In French, the possessive ma is normally used before a feminine singular noun:

• ma fiancée = my fiancée

Compare:

• mon fiancé = my fiancé (male)
• ma fiancée = my fiancée (female)

A small spelling note:

• fiancé = masculine
• fiancée = feminine

They are related, but they do not mean the same thing.

• fiancée = the person you are engaged to, if that person is female
• fiançailles = the engagement / the betrothal itself

So in the sentence:

• Ma fiancée = my fiancée
• nos fiançailles = our engagement

This is a very common thing for learners to mix up because the words look similar.

Because fiançailles is normally a plural-only noun in French. It refers to the engagement as an event or state, but French uses the plural form.

So you say:

• les fiançailles = the engagement
• nos fiançailles = our engagement

Even if English uses a singular word, French usually keeps fiançailles in the plural.

That is why the sentence has:

• nos fiançailles not
• notre fiançailles

Because the engagement belongs to both people in the couple.

The speaker says:

• Ma fiancée = my fiancée
• nos fiançailles = our engagement

Using nos makes sense because an engagement is something shared by the speaker and the fiancée.

Also, nos is the plural possessive used before a plural noun:

• nos fiançailles = our engagement

Because French possessive adjectives agree with the thing possessed, not with the owner.

Here, the owner is she:

• her stepfather / father-in-law
• her stepmother / mother-in-law

But French looks at the noun that follows:

• beau-père is masculine singular → son beau-père
• belle-mère is feminine singular → sa belle-mère

So:

• son beau-père = her stepfather / father-in-law
• sa belle-mère = her stepmother / mother-in-law

This is different from English, where her stays the same.

Yes — both words can have either meaning.

• beau-père can mean stepfather or father-in-law
• belle-mère can mean stepmother or mother-in-law

The sentence by itself does not fully remove the ambiguity. Context would tell you which meaning is intended.

Since the couple is only engaged, many learners may expect stepfather / stepmother here. But grammatically, the French words themselves are still ambiguous.

Because the verb annoncer is used like this:

• annoncer quelque chose à quelqu’un
  = to announce something to someone

In the sentence:

• nos fiançailles is the thing being announced
• à son beau-père et à sa belle-mère tells us who receives the announcement

So the structure is:

• annoncer
  • thing
• à
  • person

French often repeats the preposition before each coordinated noun phrase, especially when each noun has its own determiner.

So this is very natural:

• à son beau-père et à sa belle-mère

You may sometimes see French omit the second preposition in other contexts, but repeating it here is clear and standard.

It also helps separate the two recipients neatly:

• to her stepfather
• and to her stepmother

Literally, veut annoncer means wants to announce.

It is:

• veut = wants
• annoncer = to announce

So:

• Ma fiancée veut annoncer... = My fiancée wants to announce...

This can imply intention, but grammatically it is still wanting, not simply future time.

If French wanted to stress is going to announce, it would more likely use:

• va annoncer

Because annoncer specifically means to announce, especially when giving news.

• dire = to say / tell
• annoncer = to announce

Since an engagement is important news, annoncer is a natural verb choice.

So the sentence sounds like:

• she wants to make a formal or important announcement

rather than just:

• she wants to tell them something

It means during dinner or during the meal.

• pendant = during
• le dîner = the dinner meal

So this phrase tells you when the announcement will happen.

It suggests the announcement will happen in the course of the meal, not just generally at dinner.

Usually, in modern standard French, yes:

• petit-déjeuner = breakfast
• déjeuner = lunch
• dîner = dinner

But there are regional differences. In some places, especially in parts of the Francophone world and older usage, dîner can refer to the midday meal.

So in many learning contexts, it is safest to understand:

• le dîner = dinner

But it is good to know the word can vary by region.

The sentence follows a very common French pattern:

• Ma fiancée = subject
• veut annoncer = verb phrase
• nos fiançailles = direct object
• à son beau-père et à sa belle-mère = indirect object / recipients
• pendant le dîner = time expression

So the structure is basically:

Subject + wants to announce + thing + to whom + when

That makes this a very useful model sentence for learning how French handles:

• possessives
• infinitives after another verb
• à with recipients
• time expressions at the end of the sentence