sinseki ga kuru mae ni, edamame to karaage wo sara ni moritukete, yakisoba mo youisite okimasita.

In this sentence, 親戚が来る means the relatives come / are coming, and が marks 親戚 as the subject of 来る.

Using が is natural here because the clause 親戚が来る is just describing the event that happens before the preparation. It is not really setting 親戚 up as the main topic of the whole sentence.

If you said 親戚は来る前に, it would sound less natural in this context, because は would suggest a topic contrast, something like as for the relatives, before they come..., which is not what this sentence is focusing on.

Japanese often uses the plain form before nouns and in grammar patterns like 前に.

Here, 来る前に literally means before coming or before they come.

This is normal:

• 食べる前に = before eating
• 寝る前に = before sleeping
• 行く前に = before going

So 来る前に is not casual speech here; it is just the grammar required before 前に.

前に means before.

In 親戚が来る前に, the whole clause 親戚が来る modifies 前, so literally it is:

• the before of the relatives coming

More naturally:

• before the relatives came / before the relatives come

So the sentence is describing actions that were completed in advance of the relatives’ arrival.

With 前に, Japanese normally uses the form that refers to the action not yet having happened at that point.

So:

• 来る前に = before they come
• 食べる前に = before eating

This is different from English, where we often still say before they came in a past context.

By contrast, 後で / あとで often uses the past form:

• 来た後で = after they came
• 食べた後で = after eating

So this is a very common contrast:

• dictionary form + 前に
• past plain form + 後で / 後に

In 前に, the に marks the time point before which something happens.

So:

• 寝る前に歯を磨く = brush your teeth before sleeping
• 出かける前に鍵を確認する = check the key before going out

In your sentence, 親戚が来る前に sets the time frame for the preparations.

Because 枝豆 and 唐揚げ are joined together as one combined object by と.

So:

• 枝豆と唐揚げを = edamame and fried chicken

The を applies to both nouns together.

This is the same as:

• パンとスープを食べる = eat bread and soup
• 本とノートを買う = buy a book and a notebook

You do not need:

• 枝豆をと唐揚げを
  That would sound unnatural here.

Here, と means and.

It connects two nouns:

• 枝豆 = edamame
• 唐揚げ = Japanese fried chicken
• 枝豆と唐揚げ = edamame and fried chicken

This is the standard way to make a complete list of nouns.

盛り付ける means to arrange/serve food on a plate or dish in an appealing way.

So 皿に盛り付けて means:

• arranging/serving them on plates
• plating them

The particle に marks the place/result location:

• 皿に = onto a plate / on plates

This sounds more natural for prepared food than just saying 皿に入れて, which would simply mean putting them on a plate.
盛り付ける has a stronger nuance of presentation and serving.

The て-form is connecting actions in sequence.

Here the sentence gives a chain of actions:

1. 枝豆と唐揚げを皿に盛り付けて
2. 焼きそばも用意しておきました

So it means something like:

• I plated the edamame and karaage, and also prepared the yakisoba in advance.

The て-form often works like and, especially when listing actions that happened one after another.

も means also / too.

So:

• 焼きそばも用意しておきました = I also prepared yakisoba.

The sentence already mentioned preparing/plating 枝豆 and 唐揚げ, and then も adds 焼きそば as an additional item.

Without も, the sentence would still be grammatical, but も makes it clear that yakisoba is one more thing that was prepared.

〜ておく means to do something in advance or to do something and leave it ready for later.

So:

• 用意しました = prepared it
• 用意しておきました = prepared it in advance / got it ready ahead of time

That nuance is very important here, because the whole sentence is about getting food ready before the relatives arrive.

So 焼きそばも用意しておきました means:

• I also prepared the yakisoba ahead of time.

Because the speaker is describing completed actions.

The preparations were already done:

• plated the edamame and karaage
• prepared the yakisoba in advance

So the sentence is looking back on what was done, which is why ました is used.

Even though the clause says before the relatives come, the main action can still be in the past:

• Before the relatives came, I prepared the food.

That is exactly the idea here.

Not necessarily.

Grammatically, the sentence says:

• the edamame and karaage were plated
• the yakisoba was also prepared in advance

It does not explicitly say that the yakisoba was plated too.

Maybe it was, maybe it was not. The sentence only clearly attaches 皿に盛り付けて to 枝豆と唐揚げ.

It is polite-neutral.

The sentence uses:

• plain forms inside the clause: 来る前に
• polite ending in the main verb: おきました

This is very common in Japanese. Subordinate clauses often use plain form even in polite speech.

So the overall tone is polite because the sentence ends with 〜ました.

Yes. A good way to parse it is:

1. 親戚が来る前に
   = before the relatives come

2. 枝豆と唐揚げを皿に盛り付けて
   = plate the edamame and karaage

3. 焼きそばも用意しておきました
   = also prepared the yakisoba in advance

Put together:

• Before the relatives came, I plated the edamame and karaage, and I also prepared the yakisoba ahead of time.

This kind of chunking is very helpful for long Japanese sentences.