Plus tard, ma fiancée raconte à mon neveu et à ma nièce comment nous nous sommes rencontrés.

Not necessarily. French often uses the present tense to narrate events in sequence, especially in storytelling. So Plus tard, ma fiancée raconte... can mean something like Later, my fiancée tells... in a narrative style.

If you wanted to make the future more explicit from the speaker’s current point of view, you could say racontera instead.

So here, the present is being used in a very natural narrative way.

Because fiancée is a feminine singular noun. In French, the possessive adjective agrees with the thing possessed, not with the speaker.

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

The extra -e in fiancée marks the feminine form in writing.

They mean the same kind of relationship, but the gender is different:

• fiancé = an engaged man
• fiancée = an engaged woman

So ma fiancée tells you that the speaker is referring to a woman they are engaged to.

Because raconter here is being used with the person who receives the story.

So:

• raconter quelque chose à quelqu’un = to tell something to someone

In this sentence, what is being told is comment nous nous sommes rencontrés, and the people hearing it are mon neveu et ma nièce.

In the sentence you gave, à is actually written twice:

à mon neveu et à ma nièce

That is very natural and very clear. French often repeats the preposition before each item in a pair, especially when the phrases are a little long.

You may sometimes see only one à before both nouns, but repeating it sounds neat and unambiguous.

Because neveu is masculine and nièce is feminine.

French possessives agree with the noun that follows:

• mon neveu = my nephew
• ma nièce = my niece

It does not depend on whether the speaker is male or female.

Here, comment means how and introduces an indirect question:

• comment nous nous sommes rencontrés = how we met

This is different from a direct question such as Comment vous vous êtes rencontrés ?

In an indirect question, French keeps normal statement word order. So you do not invert anything here.

Because the two nous do different jobs:

• the first nous = the subject, we
• the second nous = the reflexive/reciprocal pronoun, roughly ourselves / each other

So:

• nous nous sommes rencontrés = we met each other

This comes from the verb se rencontrer.

Literally, it is built like that, but in real usage it usually means to meet each other when the subject is plural.

So:

• je me rencontre would sound unusual in most contexts
• nous nous rencontrons naturally means we meet each other
• nous nous sommes rencontrés means we met

This is called a reciprocal use of a reflexive verb.

Because se rencontrer is a pronominal verb. In the passé composé, pronominal verbs use être, not avoir.

So the pattern is:

• se rencontrer
• nous nous sommes rencontrés

This is the same pattern as:

• se lever → je me suis levé
• se souvenir → nous nous sommes souvenus

Because with this pronominal verb in the passé composé, the past participle agrees with the subject here.

Since the subject is nous, the participle is plural:

• rencontré = singular
• rencontrés = plural masculine or mixed group
• rencontrées = plural feminine

So rencontrés suggests either:

• the group is mixed, or
• at least one male is included, or
• masculine plural is being used as the default written form.

If the speaker and fiancée were both female, you would normally write rencontrées.

Because raconter is especially good for telling a story or narrating events.

• dire = to say / to tell
• raconter = to tell, in the sense of telling a story or recounting something

Since how we met is a story, raconter is a very natural choice.

Yes. The clause comment nous nous sommes rencontrés functions as the thing being told.

So the structure is:

• ma fiancée raconte = my fiancée tells
• à mon neveu et à ma nièce = to my nephew and my niece
• comment nous nous sommes rencontrés = how we met

In other words, she is telling them the story of how we met.