Перед встречей моя сестра хочет заплести косу.

Because перед requires the instrumental case when it means before or in front of.

• dictionary form: встреча = meeting
• instrumental singular: встречей

So:

• перед встречей = before the meeting

This is a very common pattern:

• перед уроком = before the lesson
• перед работой = before work
• перед сном = before sleep

Перед встречей means before the meeting, usually referring to the time leading up to it.

До встречи can also mean before the meeting, but it often has a slightly different feel:

• перед встречей = in the period right before the meeting
• до встречи = before the meeting / up until the meeting

In many contexts both are possible, but перед встречей feels very natural here because it sets the scene for something she wants to do in preparation.

Also, До встречи! is a fixed phrase meaning See you later!, so learners often notice that expression first.

Because коса is the direct object of заплести, so it goes into the accusative case.

• nominative: коса
• accusative singular: косу

This is normal for a feminine noun ending in -а:

• книга → книгу
• машина → машину
• коса → косу

So заплести косу literally means to braid a braid or more naturally to braid the hair into a braid.

Yes, коса has more than one meaning. Common ones include:

• braid / plait
• scythe
• sand spit in geography

Context tells you which one is meant. In this sentence, because the verb is заплести (to braid), коса clearly means braid.

So:

• заплести косу = to braid one’s hair into a braid
• not anything to do with a farming tool here

This is about aspect.

• заплести = perfective
• заплетать = imperfective
• плести = a more basic verb meaning to weave / braid, often focusing on the process

With хочет (wants), Russian often uses the perfective infinitive when the speaker means they want to complete a specific action.

So here:

• хочет заплести косу = she wants to braid her hair into a braid, with the result completed

Compare:

• хочет заплетать косу = she wants to be braiding / wants to braid habitually, which sounds less natural here
• плети косу would focus more on the process itself

Before a meeting, the idea is usually that she wants the hairstyle done, so заплести fits well.

Yes, the natural interpretation is that my sister wants to braid her own hair into a braid.

Russian often leaves this implicit when it is obvious from context. So заплести косу can naturally mean to braid one’s hair into a braid.

You could add себе:

• Моя сестра хочет заплести себе косу.

That makes it more explicit: My sister wants to braid herself a braid / wants to braid her own hair.

Without себе, the sentence is still completely normal.

Yes, Russian can often omit possessives when the meaning is already clear from context.

So both are possible:

• моя сестра хочет заплести косу = my sister wants to braid her hair into a braid
• сестра хочет заплести косу = sister wants to braid her hair into a braid

Using моя makes it explicit and can sound more natural if you are introducing her or contrasting her with someone else.

Russian generally uses possessives a bit less often than English, especially when ownership or relationship is obvious.

This is a very common Russian structure:

• хотеть
  • infinitive

So:

• хочет = wants
• заплести = to braid

Together:

• хочет заплести = wants to braid

The subject is моя сестра, so the verb хочет is in 3rd person singular:

• я хочу = I want
• ты хочешь = you want
• она хочет = she wants

Since моя сестра is she, хочет is the correct form.

No, Russian word order is fairly flexible. The sentence as given is very natural:

• Перед встречей моя сестра хочет заплести косу.

This puts the time expression first, which helps set the scene: Before the meeting...

You could also say:

• Моя сестра хочет заплести косу перед встречей.

That is also grammatical. The difference is mainly about emphasis and information flow, not basic meaning.

In the original sentence, перед встречей is fronted because it is the time frame the speaker wants to highlight first.

Literally, it is something like to braid a braid, but that sounds odd in English. The natural meaning is:

• to braid her hair into a braid
• to put her hair in a braid
• to plait her hair

Russian often uses this shorter expression because коса refers to the hairstyle itself, not just the action.

So even if the literal structure feels unusual at first, it is a normal Russian way to describe this hairstyle.