"Other than you, who else are there?"
Translation:Selain kamu, ada siapa lagi di sana?
Is there a general rule about the word order, about why "ada" needs to come before siapa lagi?
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.