Learn Hardcore Indonesian: Di museum kota, ada koleksi baru tentang sejarah keluarga nenek moyang kami. - In the city museum, there is a new collection about the history of our ancestors’ families.

Starting with Di museum kota puts the location in focus first: At the city museum, ....
You could also say Ada koleksi baru tentang sejarah keluarga nenek moyang kami di museum kota, which is also correct and natural.
Both orders are grammatically fine; it’s just a matter of emphasis and style.
Beginning with the place is very common in Indonesian storytelling and descriptions.

Why is it di museum kota and not di kota museum?

In Indonesian, the general pattern is noun + modifier, not modifier + noun.
So museum kota literally means museum (of the) city → city museum.
Kota museum would sound wrong to native speakers, because kota is functioning as a modifier here and should come after museum.

What is the difference between di museum kota and di museum di kota?

Di museum kota usually means at the city museum (a specific museum whose defining characteristic is that it’s the city’s museum).
Di museum di kota means at a museum in the city (any museum located in the city, not necessarily the official city museum).
So museum kota is like a fixed type of museum, while museum di kota just describes the museum’s location.

What exactly does ada do in this sentence?

Ada here works like there is / there are in English.
Ada koleksi baru = There is a new collection.
Without ada, the sentence would feel incomplete or sound like a fragment.
Indonesian often uses ada to introduce the existence or presence of something in a place.

Is the comma after kota necessary: Di museum kota, ada koleksi baru ...?

The comma is common and stylistically nice, but not strictly required in everyday writing.
It marks a pause after the fronted location phrase Di museum kota.
You can also write Di museum kota ada koleksi baru tentang ... without a comma, and it is still correct and natural.

Why is it koleksi baru, not baru koleksi?

In Indonesian, adjectives usually come after the noun they describe.
So koleksi baru = new collection.
Baru koleksi is not grammatical in this context; it would sound wrong.
This pattern also appears in museum kota (city museum), buku sejarah (history book), rumah besar (big house), and so on.

How would I make koleksi baru clearly plural, like new collections?

Indonesian often leaves plural implied from context, so koleksi baru can mean a new collection or new collections.
To make it clearly plural, you can say:

beberapa koleksi baru = several new collections
banyak koleksi baru = many new collections
koleksi-koleksi baru = new collections (reduplication emphasizes plurality, often more written/formal or emphatic).

What does tentang mean here, and can I replace it with something else?

Tentang means about / regarding.
Koleksi baru tentang sejarah ... = a new collection about the history ....
You can often replace tentang with mengenai or soal:

koleksi baru mengenai sejarah ... (a bit more formal)
koleksi baru soal sejarah ... (more casual/conversational).
All are understandable; tentang is the safest, neutral choice.

How is sejarah keluarga nenek moyang kami structured? What modifies what?

The structure is basically:
sejarah (history)
→ of keluarga (family)
→ of nenek moyang (ancestors)
→ kami (our).

So it’s the history of the family of our ancestors.
In smoother English we might say the family history of our ancestors or the history of our ancestral family.

Is keluarga nenek moyang redundant? Doesn’t nenek moyang already mean ancestors?

Nenek moyang does already mean ancestors, so keluarga nenek moyang kami can sound a bit layered or poetic.
It can suggest things like:

the broader family lines of our ancestors
the family relationships among our ancestors
or simply emphasize our ancestral family in a fuller way.

In more neutral wording, you could simplify it to sejarah nenek moyang kami (the history of our ancestors).

Why is it kami and not kita in nenek moyang kami?

Kami means we / our but excluding the listener.
Kita means we / our including the listener.
Here, nenek moyang kami implies our ancestors in the sense of the group being talked about, not automatically including the person being spoken to.
If you wanted to explicitly include the listener’s ancestors too, you could say nenek moyang kita.

Can I change the word order in the second part, for example: Ada koleksi baru di museum kota tentang sejarah keluarga nenek moyang kami?

Yes, that word order is perfectly grammatical and natural.
These are all acceptable, with slightly different emphasis:

Di museum kota, ada koleksi baru tentang sejarah keluarga nenek moyang kami. (location emphasized first)
Ada koleksi baru di museum kota tentang sejarah keluarga nenek moyang kami. (existence of a new collection emphasized first)
Ada koleksi baru tentang sejarah keluarga nenek moyang kami di museum kota. (new collection and its topic emphasized, location last).

Indonesian allows fairly flexible placement of location phrases, as long as the meaning stays clear.