Quand mes cousines se voient, elles se parlent l’une à l’autre pendant des heures.

Here se is reciprocal, not just reflexive.

• se voir = to see each other
• se parler = to talk to each other

So mes cousines se voient means the cousins see one another, and elles se parlent means they talk to one another.

In English, we often use each other. In French, that idea is often built into the verb with se.

Because l’une à l’autre adds emphasis and clarity.

• elles se parlent already means they talk to each other
• elles se parlent l’une à l’autre means something like they talk to one another

So this phrase is not strictly necessary, but it reinforces the reciprocal idea.

Also, with parler, French often likes to make the relationship between the people extra clear, especially in fuller or more careful style.

Because cousines is feminine plural.

• l’une à l’autre = one to the other for two feminine people
• l’un à l’autre = masculine or mixed-gender version

Since both people are female cousins, French uses the feminine form une.

In this sentence, Quand can be understood as when or whenever, depending on context.

Because the sentence is in the present tense and describes something habitual, the meaning is closer to:

• Whenever my cousins see each other, they talk to each other for hours

So it is not just one single occasion; it sounds like a general pattern.

French often uses the present tense for general truths, habits, and repeated actions.

This sentence is describing what usually happens:

• when they see each other,
• they talk for hours.

So the present tense works naturally in both clauses.

Because the subject is plural: mes cousines / elles.

The verb voir in the present tense changes like this:

• je vois
• tu vois
• il/elle voit
• nous voyons
• vous voyez
• ils/elles voient

So elles se voient is the correct form for they see each other.

A useful thing to notice: although voient is spelled with -ent, that ending is usually not pronounced.

Because elles is also third person plural.

The verb parler in the present tense goes:

• je parle
• tu parles
• il/elle parle
• nous parlons
• vous parlez
• ils/elles parlent

So elles se parlent is simply they talk to each other.

Again, the -ent in parlent is normally silent.

Because the two verbs do not work the same way.

• voir quelqu’un = to see someone
  This takes a direct object.
• parler à quelqu’un = to speak to someone
  This takes an indirect object with à.

So:

• elles se voient = they see each other
• elles se parlent = they speak to each other

And the expanded phrase l’une à l’autre matches that à relationship: one to the other.

Because the second clause needs its own subject.

French usually repeats the subject before a new finite verb, especially after a clause like Quand...

So:

• Quand mes cousines se voient, elles se parlent...

is the normal structure.

English also does this:

• When my cousins see each other, they talk for hours.

You would not normally leave out they, and French does not leave out elles here either.

It means for hours.

• pendant = for (in the sense of duration)
• des heures = hours

So pendant des heures describes how long they talk.

It suggests a long stretch of time, without giving an exact number.

Because French uses des here for an indefinite plural quantity.

• des heures = hours, for hours, for many hours

You are not talking about specific hours on a clock, so les heures would not fit.

And French normally needs an article before a plural noun, so just heures by itself would not work here.

It can be both, depending on context.

• Elle se voit dans le miroir.
  = She sees herself in the mirror.
  This is truly reflexive.

• Mes cousines se voient.
  = My cousins see each other.
  This is reciprocal.

So the same se form can mean oneself or each other. The context tells you which meaning is intended.

Yes. You can simply say:

Quand mes cousines se voient, elles se parlent pendant des heures.

That still means When/Whenever my cousins see each other, they talk to each other for hours.

Adding l’une à l’autre just makes the reciprocal meaning more explicit and a little more emphatic.