Learn Hardcore Indonesian: Keluarga kami menanggung biaya sekolah adik laki-laki saya. - Our family bears my younger brother's school expenses.

keluarga kami = our family (not including the listener)
Used when the listener is not part of that family.
→ In this sentence, it implies the listener is an outsider to the speaker’s family.
keluarga kita = our family (including the listener)
Used when you and the listener share the same family (e.g. talking to a sibling).
keluarga saya = my family
Grammatically fine, but feels a bit more individual, like emphasizing “my” rather than “our.”

In your sentence, keluarga kami is natural if you are talking to someone who is not part of your family.

Why is it adik laki-laki saya and not adik saya laki-laki?

Indonesian typically puts descriptors after the main noun in a fixed order:

Main noun
Gender/descriptor (if any)
Possessive pronoun

So:

adik (younger sibling)
laki-laki (male)
saya (my)

→ adik laki-laki saya = “my younger brother”

Putting it as adik saya laki-laki sounds unnatural and broken, like saying “my sibling, male” in a chopped-up way. Native speakers don’t structure it that way.

Do I have to say adik laki-laki, or can I just say adik saya?

You don’t have to say laki-laki. Options:

adik saya = my younger sibling (gender not specified)
adik laki-laki saya = my younger brother
adik perempuan saya = my younger sister

In many contexts, adik saya is enough if gender is clear from context or not important. In your sentence, adik laki-laki saya just makes it explicit that the sibling is male.

What exactly does menanggung mean here? Why not just use membayar?

Both are about paying, but they differ in nuance:

membayar biaya sekolah = to pay the school fees (simple act of payment)
menanggung biaya sekolah = to bear/shoulder the cost, to take responsibility for paying

menanggung suggests ongoing responsibility or obligation, not just a one-time transaction.
So:

Keluarga kami membayar biaya sekolah adik laki-laki saya.
Our family pays my younger brother’s school fees.
Keluarga kami menanggung biaya sekolah adik laki-laki saya.
Our family is responsible for covering my younger brother’s school expenses.

Both are correct; the original sentence feels a bit more formal and responsibility-focused.

What is the difference between biaya sekolah and something like uang sekolah?

Both can be used for school-related costs, but:

biaya sekolah = school expenses/costs (more general, slightly more formal)
uang sekolah = school fee(s)/school money (sounds more like the actual money paid, a bit more everyday/colloquial)

In many contexts they overlap:

menanggung biaya sekolah – bears the cost of schooling
membayar uang sekolah – pays the school fee

Your sentence uses biaya sekolah because it fits well with the more formal verb menanggung.

How does biaya sekolah adik laki-laki saya work grammatically? There is no “of” or “for.”

Indonesian often expresses possession and “of/for” relationships simply by putting nouns next to each other (noun stacking), in this kind of rightward chain:

biaya = cost
biaya sekolah = cost of school / school expenses
biaya sekolah adik laki-laki saya = school expenses of my younger brother

So instead of “the school expenses of my younger brother,” Indonesian just stacks the nouns:
biaya sekolah adik laki-laki saya.

You could also say:

biaya sekolah untuk adik laki-laki saya = costs for my younger brother’s schooling

This is also correct, just a bit more explicit with untuk (for).

Why is it kami and not kita in this sentence?

Indonesian distinguishes two kinds of “we”:

kami = we / us, excluding the listener
kita = we / us, including the listener

In your sentence:

Keluarga kami… implies “our family” but you are talking to someone outside the family, so they are not part of kami.

If you were talking to your sibling (someone in the same family), you’d naturally say:

Keluarga kita menanggung biaya sekolah adik laki-laki kita.
Our family bears the cost of our younger brother’s schooling.

How do I know the tense? Does this mean “pays,” “paid,” or “will pay”?

Indonesian verbs like menanggung do not change form for tense. The sentence by itself is time-neutral and could mean:

Our family pays my younger brother’s school expenses.
Our family paid my younger brother’s school expenses.
Our family will pay my younger brother’s school expenses.

Context or time words make it clear:

sudah / telah menanggung = has/ already paid
sedang menanggung = is currently bearing/paying
akan menanggung = will bear/pay
dulu menanggung = used to bear/pay (in the past)

Without such markers, listeners infer the time from the conversation context.

Is the whole sentence formal, informal, or neutral? Would people say this in conversation?

The sentence is neutral, leaning slightly formal because of menanggung biaya. It’s very natural in both spoken and written Indonesian, especially when discussing responsibilities or finances.

More casual variants might be:

Keluarga kami yang bayar uang sekolah adik saya.
It’s our family that pays my younger sibling’s school fees.
Keluarga kami biayai sekolah adik saya.
Our family funds my younger sibling’s schooling.

But your original sentence is perfectly acceptable in everyday speech, especially in a slightly careful or polite context.

Could I leave out kami and just say Keluarga menanggung biaya sekolah adik laki-laki saya?

You can say that, but the meaning changes slightly:

Keluarga kami menanggung… = Our family bears the cost. (clearly “our”)
Keluarga menanggung… = The family bears the cost. (which family? It’s vague)

Without kami, keluarga could refer to:

your family (if context already makes that clear),
or any “the family” being discussed.

If you want to clearly say “our family,” you should keep kami (or kita, depending on whether the listener is included).