"or" usage for choices In English (and Spanish), "or" ("o") can be between things.  In Spanish, if negating, both items are preceded by "ni".

I'm looking at sample sentences in Chinese, and sometimes I see

或____或____

but in others, the first 或 is not there.

Is this arbitrary, or is there a simple or complex rule to know which to use?

I was about to write 我沒有或姐姐或哥哥。
"____或____" = "____or____"


Example : 

我沒有姐姐或(者)哥哥 (I don't have sister or brother)


  "是____或是____" = "is ____or____"


Example : 

來的是姐姐或是哥哥都不要緊 (It doesn't matter the one who comes is sister or brother)

~


  "或____或____" = "either____ or____"


Example:

全部隊員或(者)死或(者)傷，我们已經沒法再戦 (All team members either died or injured, we can fight no longer) 


  "或是____或是____" = "is either____ or____"


Example:
來的或是姐姐, 或是哥哥, 總之不會是父母 (The one who comes is either sister or brother, in short, will not be parents)


  I was about to write 我沒有或姐姐或哥哥。


As user6065 stated:

我既沒有姐姐也没有哥哥 (I have neither sister nor brother)

我既有姐姐也有哥哥 (I have sister and also brother)