ane ha, gyouza wo yakinagara, edamame mo sara ni moritukete ita.

姉 is the plain dictionary-style word for older sister.

In Japanese, when talking about your own family members to someone else, you often use the plain kinship term:

• 姉 = my older sister
• 母 = my mother
• 父 = my father

By contrast, 姉さん is more like older sister / your older sister / someone else’s older sister, or a more direct address.

So in a sentence like this, 姉は... naturally means As for my older sister, ...

は marks 姉 as the topic of the sentence.

So 姉は means something like:

• As for my older sister, ...
• My older sister, ...

It does not mark the grammatical subject in the same way English does. It sets up what the sentence is about.

There can also be a slight contrastive feeling depending on context, like:

• As for my older sister, she was...

But in many cases, it is just the normal topic marker.

焼きながら comes from:

• 焼く = to grill, fry, cook by heat
• verb stem 焼き
• ながら = while doing

So:

• 焼きながら = while grilling/frying/cooking

The pattern is:

• verb stem + ながら

Examples:

• 音楽を聞きながら勉強する = study while listening to music
• 歩きながら話す = talk while walking

In your sentence, it means she was doing one action while doing another.

Yes, normally yes.

With AしながらBする, the subject is usually the same person for both actions.

So this sentence means that the older sister was:

• frying/cooking the gyoza
• and at the same time plating the edamame

It would sound unnatural to use ながら if one person did the first action and someone else did the second.

Grammatically, the action after ながら is the main clause.

So in:

• 餃子を焼きながら、枝豆も皿に盛り付けていた。

the main predicate is:

• 盛り付けていた = was plating / had been plating

And 焼きながら gives the background simultaneous action:

• while frying gyoza

That does not mean plating is more important in real life; it just means that structurally the sentence ends with that action.

を marks the direct object of 焼く.

So:

• 餃子を焼く = to cook/fry/grill gyoza

Even though 焼く can literally mean to burn in some contexts, in cooking it often means things like:

• grill
• fry
• cook on a pan
• bake

With 餃子, 焼く is very natural, because pan-fried gyoza are commonly called 焼き餃子.

Good question. 焼く has a broader range than any one English word.

Depending on context, it can mean:

• burn
• grill
• roast
• bake
• pan-fry / cook by heating

With 餃子, it commonly refers to cooking gyoza in a pan, so English translations may say:

• frying gyoza
• cooking gyoza
• pan-frying gyoza

So yes, even though the core idea is heating something until it’s cooked/browned, the best English word changes by context.

も means also / too.

So:

• 枝豆も = edamame too / also the edamame

It shows that in addition to frying the gyoza, she was also doing something with the edamame.

There is a mild sense of addition:

• She was frying gyoza, and also plating the edamame.

If it were just 枝豆を, the sentence would be more neutral and would not emphasize the also.

Because に marks the destination/place where something is put.

• 皿に盛り付ける = arrange/serve onto a plate

Here, the edamame is being placed onto the plate, so 皿に is natural.

Compare:

• 本を机に置く = put a book on the desk
• 料理を皿に盛る = serve food onto a plate

Using を with 皿 here would not fit this verb in the same way.

盛り付ける means to serve, plate, or arrange food on a dish/plate, often with some attention to presentation.

It is more specific than just put.

So:

• 枝豆を皿に盛り付ける = to plate the edamame on a dish

It suggests the food is being prepared for serving, not just casually dropped somewhere.

A related word is:

• 盛る = to heap up / serve food onto a plate or bowl

盛り付ける often sounds a little more like arranging for presentation.

盛り付けていた is the past progressive / past ongoing form.

• 盛り付けた = plated / served
• 盛り付けていた = was plating / had been plating

In this sentence, ていた shows that the action was in progress at that point in the past.

That matches well with ながら, because the sentence is describing simultaneous ongoing actions:

• she was frying gyoza
• while she was also plating edamame

If you said 盛り付けた, it would sound more like a completed action rather than an ongoing scene.

Japanese often omits information that is already clear from context.

Here, the subject 姉 applies to both actions:

• 姉は、餃子を焼きながら、
• 枝豆も皿に盛り付けていた。

So the sentence does not need to repeat 姉は again.

In English, we often repeat the subject more explicitly, but Japanese regularly leaves it out once it is understood.

Not naturally. ながら strongly suggests the actions overlap.

So the basic meaning is:

• she was frying gyoza while plating edamame

That said, real-life simultaneity can be loose. It does not have to mean both actions happened in the exact same millisecond. It can describe a scene where she was handling both tasks as part of the same ongoing process.

But it definitely does not sound like:

• first she fried the gyoza, then later she plated the edamame

For that, Japanese would use a different structure.

Yes, there can be a nice subtle structure here.

• 姉は sets the topic: as for my older sister
• 枝豆も adds: the edamame too / also the edamame

So the sentence has a feeling like:

• As for my older sister, while she was frying gyoza, she was also plating edamame.

The も helps show she was doing more than one thing, which fits the multitasking feel of the sentence.

Yes. 皿 is a general word for a plate or dish.

Depending on context, English could say:

• plate
• dish

In this sentence, plate is probably the most natural choice for many learners, but dish is also fine.

Yes, it is natural.

It describes a scene in a very ordinary way:

• topic: older sister
• simultaneous action: 焼きながら
• added item: 枝豆も
• ongoing past action: 盛り付けていた

It sounds like someone narrating what was happening in the kitchen.

A smooth English rendering might be:

• My older sister was frying gyoza while also plating edamame.
• While my older sister was cooking gyoza, she was also serving edamame onto a plate.

Both capture the sense well.