Моя сестра хочет заплести косу перед выходом из дома.

Because сестра is a feminine noun, and the possessive pronoun my has to agree with it in gender, number, and case.

• masculine: мой брат
• feminine: моя сестра
• neuter: моё окно
• plural: мои друзья

Here сестра is feminine singular nominative, so моя is the correct form.

After хотеть (to want), Russian normally uses an infinitive to say what someone wants to do.

So:

• хочет заплести = wants to braid
• literally: wants to braid a braid / wants to make a braid

This is the same pattern as in:

• Я хочу поесть = I want to eat
• Она хочет уйти = She wants to leave
• Мы хотим купить книгу = We want to buy a book

So хочет + infinitive is completely standard.

This is about verb aspect.

• плести = to weave / braid, more general or process-oriented
• заплетать = imperfective, to braid / to be braiding / to braid regularly
• заплести = perfective, to braid something completely, to finish braiding

In this sentence, the idea is that she wants to complete the action before leaving the house. That is why заплести is natural.

Compare:

• Она заплетает косу. = She is braiding her braid / she braids her hair into a braid.
• Она хочет заплести косу. = She wants to braid her hair into a braid (and get it done).

After хотеть, both imperfective and perfective infinitives are possible, but they give different shades of meaning. Here the perfective fits because the result matters.

Here коса means a braid or plait of hair.

Russian коса can have more than one meaning depending on context, for example:

• коса = braid/plait
• коса = scythe
• песчаная коса = sand spit

In this sentence, because of заплести and сестра, it clearly means a braid of hair.

So заплести косу means to braid one’s hair into a braid.

Because коса is the direct object of заплести, so it must be in the accusative case.

The noun коса is feminine and follows the common -а declension pattern:

• nominative: коса
• accusative: косу

Compare:

• Она видит сестру.
• Она читает книгу.
• Она хочет заплести косу.

So косу is just the expected accusative singular form.

Russian often expresses this idea differently from English.

English usually says:

• She wants to braid her hair

Russian often says:

• заплести косу = literally to braid a braid

This means to arrange the hair into a braid. It is an idiomatic way to describe the hairstyle result.

You could also say things with волосы, but that changes the emphasis:

• заплести волосы = to braid the hair
• заплести косу = to make a braid / braid the hair into a braid

The sentence focuses on the hairstyle itself: a braid.

Normally, it is understood as her own, because that is the most natural interpretation in context.

Russian often leaves out words like себе (for herself / to herself) when the meaning is obvious.

So:

• Моя сестра хочет заплести косу
  naturally means
• My sister wants to braid her hair into a braid

If you wanted to make the reflexive idea more explicit, you could say:

• Моя сестра хочет заплести себе косу.

That sounds a bit more explicit, but the original sentence is perfectly normal without себе.

Because the preposition перед in the sense of before takes the instrumental case.

The noun выход (exit / leaving) is masculine, so in the instrumental singular it becomes выходом.

• nominative: выход
• instrumental: выходом

So:

• перед выходом = before leaving / before the exit / before going out

More examples:

• перед уроком = before class
• перед работой = before work
• перед сном = before sleep / before going to bed

Russian often uses a preposition + verbal noun where English might prefer a clause or an -ing form.

So:

• перед выходом из дома = before leaving the house

Literally, it is closer to:

• before the leaving from the house

This is very natural in Russian. You could also say:

• прежде чем выйти из дома = before going out of the house

Both are correct, but перед выходом из дома is compact and natural.

Because из means movement out of the inside of something.

• из дома = out of the house / from inside the house

This fits the idea of leaving a building.

Compare:

• из дома = from inside the house
• от дома = from the house, away from the house as a point/location
• с дома is generally not standard here

Also, из requires the genitive case, so:

• дом → дома

That is why we get из дома.

It is genitive singular, because the preposition из requires the genitive.

• nominative: дом
• genitive: дома

So:

• из дома = out of the house / from the house

This is a very common pattern:

• из школы = from school
• из комнаты = from the room
• из города = from the city

Yes. Russian word order is flexible, though different orders can slightly change emphasis.

Original:

• Моя сестра хочет заплести косу перед выходом из дома.

Other possible orders:

• Перед выходом из дома моя сестра хочет заплести косу.
• Моя сестра перед выходом из дома хочет zapлести косу.
• Косу моя сестра хочет заплести перед выходом из дома.

The original version is neutral and natural. Putting перед выходом из дома first gives more emphasis to the time frame.

Yes. Singular косу suggests one braid.

If you wanted to say braids in the plural, you could say:

• заплести косы = to braid the hair into braids

So the sentence as written suggests a hairstyle with one braid.

A helpful stress pattern is:

• Моя́ сестра́ хо́чет заплести́ ко́су пе́ред вы́ходом из до́ма.

A few key stresses:

• моя́
• сестра́
• хо́чет
• заплести́
• ко́су
• пе́ред
• вы́ходом
• до́ма

For an English speaker, one especially useful thing to notice is that заплести́ is stressed on the last syllable, while ко́су is stressed on the first.