Is there any reason why abang requires seorang but saudari does not?
There isn't. .... tapi saya punya abang' should work too.
Tapi .... tetapi should be accepted too
i do not have sisters but i a have an older brother
Is there a term for older sister? Why is there older brother but no gender for so many other family designations?
Saudara/saudari come from Sanskrit, so they specify sex but not age, as do European languages. Terms from Malay and Javanese specify age, but not gender. As for why that is the case, I doubt we'll ever know without a time machine.