Kakak perempuan saya memasang bros kecil di kerah blus birunya sebelum rapat.

Kakak means older sibling. By itself, it does not tell you whether the sibling is male or female.

Adding perempuan makes it specifically female, so kakak perempuan means older sister.

Then saya means my here, so:

kakak perempuan saya = my older sister

Yes. Kakak saya can mean my older sibling, and in many contexts it is understood as my older sister if the gender is already clear.

Indonesian often leaves out information that the listener can figure out from context. So:

• kakak saya = my older sibling / my older sister / my older brother
• kakak perempuan saya = my older sister, explicitly

If you want to be precise, kakak perempuan saya is clearer.

In Indonesian, possessors usually come after the noun.

So:

• rumah saya = my house
• ibu saya = my mother
• kakak perempuan saya = my older sister

This is a very common pattern. English says my house, but Indonesian says more literally house my.

Memasang comes from the root pasang, which has the general idea of putting something in place, attaching, installing, or setting up.

In this sentence, memasang bros means something like:

• putting on a brooch
• attaching a brooch
• fastening a brooch

It is natural Indonesian. A more specific verb you may also hear is menyematkan, which is especially good for pinning something like a brooch or badge onto clothing.

So:

• memasang bros = natural and common
• menyematkan bros = a bit more specific, like pinning a brooch on

Because in Indonesian, adjectives usually come after the noun.

So:

• bros kecil = small brooch
• blus biru = blue blouse
• rumah besar = big house

This is the normal Indonesian word order:

noun + adjective

Here di is a preposition meaning in, on, or at, depending on context.

So di kerah means on the collar.

A very important point: this di is separate from the passive prefix di-.

• di kerah = on the collar
• dipasang = installed / attached, with the passive prefix

If there is a space, it is usually the preposition di.
If there is no space and it is attached to a verb, it is usually the passive prefix di-.

Indonesian often connects nouns directly, without a separate word for of.

So:

• kerah blus = blouse collar
• more naturally in English: the collar of the blouse

In this sentence:

kerah blus birunya
literally follows the pattern:

collar + blouse + blue + -nya

So the whole phrase means:

the collar of her blue blouse
or the collar of the blue blouse

English needs of, but Indonesian usually does not.

The suffix -nya often marks something as specific, definite, or already known from context. It can also indicate possession such as his, her, or its.

In blus birunya, the -nya is attached to the last word of the noun phrase, biru, not because it belongs only to the color word, but because Indonesian often places -nya at the end of the whole phrase.

So:

• blus biru = a blue blouse / blue blouse
• blus birunya = the blue blouse / her blue blouse

In this sentence, because the subject is my older sister, English naturally understands it as her blue blouse.

No. That is a very common first impression, but -nya is more flexible than that.

It can mean things like:

• his
• her
• its
• the
• sometimes that one, depending on context

For example:

• bukunya = his book / her book / the book
• rumahnya = his house / her house / the house

In your sentence, blus birunya is best understood as her blue blouse, but that comes from context, not from -nya alone.

Indonesian usually does not use articles the way English does.

English requires choices like:

• a brooch
• the brooch
• a meeting
• the meeting

Indonesian often leaves that unstated and lets context do the work.

So:

• bros kecil could mean a small brooch or the small brooch
• rapat could mean a meeting or the meeting

If Indonesian wants to make something more definite, it may use context, word order, or forms like -nya.

That is one reason blus birunya feels more definite than just blus biru.

Sebelum means before, and rapat here means meeting.

So sebelum rapat means:

• before the meeting
• before a meeting

Which one is best depends on context, because Indonesian does not force the same article choice that English does.

Also, rapat can mean other things in other contexts, such as tight or close together, but in this sentence it is clearly the noun meeting.

Yes, it is natural. A native speaker would understand it easily.

You could also say it in slightly different ways, depending on style:

• Kakak perempuan saya menyematkan bros kecil di kerah blus birunya sebelum rapat.
  This sounds a bit more specifically like pinning the brooch onto the blouse.

• Sebelum rapat, kakak perempuan saya memasang bros kecil di kerah blus birunya.
  Same meaning, but with before the meeting moved to the front for emphasis.

So the original sentence is good, and the alternatives are just stylistic variations.