ane ha, yakitori wo tabenagara, terebi de yakyuu wo mite ita.

Yes, in a normal sentence like this, 姉 usually means older sister, and very often it refers to the speaker’s own older sister.

A useful family-word nuance:

• 姉 = my older sister when talking to someone else
• お姉さん = someone else’s older sister, or a polite/friendly way to say older sister

So if a learner sees 姉は..., it is very natural to understand it as As for my older sister...

は marks the topic of the sentence.

So 姉は means something like:

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

It tells you who the sentence is about. It does not directly mean the subject in the same way English does; it sets the topic first, and then the speaker says something about that topic.

If this sentence used が instead, the nuance would be different. が would sound more like identifying or emphasizing the older sister as the one doing the action. But は is the most natural choice here for a simple statement about her.

を marks the direct object of the verb.

Here, the verb is 食べる (to eat), so the thing being eaten is marked with を:

• 焼き鳥を食べる = eat yakitori

So 焼き鳥を tells you that yakitori is what the older sister is eating.

ながら means while doing.

It attaches to the verb stem:

• 食べる → 食べ
• 食べながら = while eating

A very important point: with ながら, the subject is normally the same person for both actions.

So in this sentence, the older sister is:

• eating yakitori
• while watching baseball on TV

It would sound odd to use ながら if one person did the first action and a different person did the second.

Also, the ながら action is often a kind of background or accompanying action, while the main clause gives the main event.

No, that would not be the normal reading.

With Vます-stem + ながら, the same subject usually does both actions. So the natural interpretation is:

• the older sister was eating yakitori
• and at the same time, she was watching baseball on TV

If Japanese wanted to clearly show different people doing the actions, it would normally use a different structure.

Here, で marks the means / medium.

So テレビで野球を見る means:

• watch baseball on TV

In this sentence, テレビ is not the thing being watched as an object in the grammatical sense. The real object of 見る is 野球, and テレビで tells you the medium through which she watched it.

Compare:

• テレビで見る = watch it on TV
• 映画を見る = watch a movie

So で here is like by means of / via / on.

Because 見る normally takes a direct object marked by を.

So:

• 野球を見る = watch baseball

English learners sometimes wonder why it is not 野球が here. The answer is that 見る is a transitive verb, and the thing you watch is usually marked with を.

So in this sentence:

• 野球を見ていた = was watching baseball

見ていた is the past form of 見ている, and here it shows an ongoing action in the past.

So:

• 見た = watched / saw
• 見ていた = was watching

That fits the sentence well because the speaker is describing a scene in progress:

• the older sister was eating
• while she was watching baseball on TV

Using 見た would sound more like a simple completed action, while 見ていた gives the feeling of an action that was underway at that time.

No. 食べながら only tells you that the action of eating was happening at the same time as the other action.

It does not tell you whether:

• she finished the yakitori
• she ate only some of it
• she kept eating afterward

It simply means while eating.

Japanese sentence order is more flexible than English, but the basic pattern is that the main verb comes near the end.

This sentence is organized like this:

• 姉は = topic
• 焼き鳥を食べながら = while eating yakitori
• テレビで = on TV
• 野球を = baseball
• 見ていた = was watching

A natural English translation has to rearrange the parts, but in Japanese this order is very normal.

A good way to think of it is that Japanese often builds the sentence piece by piece and saves the main verb for the end.

Not grammatically, no. The commas mainly help readability and show a pause.

So:

• 姉は、焼き鳥を食べながら、テレビで野球を見ていた。

and

• 姉は焼き鳥を食べながらテレビで野球を見ていた。

mean the same thing.

In learner materials, commas are often added to make the structure easier to see.