imouto ha hasami de kami wo tiisaku kitte kara, huutou ni iremasita.

は marks 妹 as the topic of the sentence: as for my younger sister / my younger sister, ...

In a sentence like this, は sounds natural because the speaker is simply telling what the younger sister did.

If you used が, it would put more focus on who did the action, as if answering a question like Who cut the paper and put it in the envelope?

So here:

• 妹は = As for my younger sister, ...
• 妹が = It was my younger sister who ... / stronger focus on the subject

Usually, yes, in many contexts 妹 by itself is understood as my younger sister or the younger sister relevant to the conversation.

Japanese often leaves out words like my, the, and a when the meaning is clear from context.

So 妹は could be understood as:

• My younger sister
• My little sister
• The younger sister

depending on the situation.

Also, 妹 specifically means younger sister. An older sister is 姉.

The particle で here marks the tool or means used to do the action.

So:

• はさみで切る = cut with scissors

This is a very common use of で:

• 箸で食べる = eat with chopsticks
• ペンで書く = write with a pen
• ナイフで切る = cut with a knife

So はさみで tells you what was used to cut the paper.

を marks the direct object of the verb, meaning the thing directly affected by the action.

Here, the verb is 切る (to cut), and the thing being cut is 紙 (paper), so:

• 紙を切る = cut the paper

That is why 紙 takes を.

小さく is the adverbial form of the い-adjective 小さい (small).

• 小さい = small
• 小さく = in a small way / into small pieces / small

In this sentence, it modifies 切って, so the meaning is something like:

• cut the paper small
• cut the paper into small pieces

This is a very common pattern:

• 早い → 早く = quickly / early
• 大きい → 大きく = big / largely / into something bigger
• 小さい → 小さく

So 紙を小さく切って means the paper was cut so that it became small.

This is a very important difference.

• Vてから = after doing V
• Vたから = because V did / because V was

So:

• 切ってから、封筒に入れました = After cutting it, she put it in the envelope
• 切ったから、封筒に入れました would sound like Because she cut it, she put it in the envelope, which is a different meaning

Here, から does not mean because. It is part of the grammar pattern てから, which means after.

〜てから means after doing ..., and it shows that one action happens first, then another action happens next.

In this sentence:

1. 紙を小さく切って
2. そのあとで封筒に入れました

So the order is clear:

• first: cut the paper into small pieces
• second: put it into the envelope

This pattern is very common:

• ご飯を食べてから勉強します = I’ll study after eating
• 家に帰ってから寝ました = I slept after returning home

Because 切って is in the て-form, which is being used to connect actions, not to end the sentence.

The final verb in the sentence is 入れました, and that is where the tense and politeness are shown.

So:

• 切ってから = after cutting
• 入れました = put in

Japanese often puts the full tense/politeness marking only on the final verb of the sentence.

With 入れる (to put in / insert), に marks the destination or place something goes into.

So:

• 封筒に入れる = put into an envelope

This is similar to:

• 箱に入れる = put into a box
• かばんに入れる = put into a bag

So に here tells you the target location of the movement.

Because the object is understood from context and has been omitted.

Earlier in the sentence, we already have 紙を with 切る. After that, it is natural to understand that the same thing—the paper—is what got put into the envelope.

So the full idea is:

• 妹は はさみで 紙を 小さく切ってから、紙を 封筒に 入れました。

But repeating 紙を again is unnecessary, so Japanese often leaves it out.

This kind of omission is extremely common in Japanese.

Yes. In natural English, 小さく切る is often best translated as:

• cut into small pieces
• cut up small
• cut finely
  depending on context

Literally it is something like cut so that it becomes small, but natural translation often sounds better with into small pieces.

So in this sentence, the idea is not just that the act of cutting was small, but that the paper became small after being cut.

Japanese normally does not use articles like the or a, and nouns usually do not change for singular/plural in the same way English nouns do.

So:

• 紙 can mean paper, a paper, the paper, or sometimes papers
• 封筒 can mean an envelope or the envelope

The exact meaning depends on context.

That is why Japanese sentences can feel less specific on the surface than English ones, even though the intended meaning is usually clear in context.

Not grammatically essential. The comma after から just shows a pause and makes the sentence easier to read.

So these are both basically fine:

• 妹ははさみで紙を小さく切ってから、封筒に入れました。
• 妹ははさみで紙を小さく切ってから封筒に入れました。

The comma simply helps separate the two actions a little more clearly.