"Other than you, who else are there?"

Translation: Selain kamu, ada siapa lagi di sana?

[JamesTWils]

Is there a general rule about the word order, about why "ada" needs to come before siapa lagi?

[Rick392366]

I'm not sure about the general rules, but it's said like that.
You can even omit 'ada' , because it's obvious that you're talking about presence/location.

Selain kamu, ada siapa lagi di sana?
Selain kamu, siapa lagi di sana?
Selain kamu, siapa lagi (yang ada) di sana?

They all have the same meaning.

[Ria___]

I would have thought that "Selain kamu, ada siapa lagi?" would be correct as well as the "there is/are" is implied in the "ada".