Part of the Weebing 101 Series
The way Japanese refer to family members is a bit different than we do in English. Do you remember watching Yu-Gi-Oh! and wanting to punch gravel-voiced Mokuba in the face every time he said “Big brother!” because it sounded so weird and not right? No? Just me? Well anyway, Mokuba wasn’t just a weird annoying kid, he was a Japanese kid. Similarly to how we refer to our parents as Mom and Dad rather than by their first names, younger siblings will refer to older siblings in a similar way.
Big Brother/Big Sister: This can be said several ways, but all have a root of “Ni” or “Ne” respectively. Most common is going to be “Ani” (when talking about YOUR older brother) but since we’re watching anime it’s going to be Nii-san, Onii-san, or you can swap that honorific with whatever really, leading to the infamous cute younger sister yelling “Onii-chan!” If your older brother is distinguished, you can even make it “Onii-sama.”
For big sister change those “i”s to “e”s. “Ane,” “Nee-san,” “Onee-san,” “Onee-chan,” etc. It isn’t uncommon for younger kids to address older non-relatives as Onii-san or Onee-san as well, especially if they don’t know the person’s name. You’ll often see nii-san or nee-san used as honorifics when addressing/referring to someone who isn’t a relative, but is *like* family, such as in Fate/Stay Night (Fuji-nee-san) and the Monogatari Series (Koyomi-onii-chan).
Just as you can address those you don’t know as “onii-san” or “onee-san,” if they’re significantly older you can call them Aunt or Uncle, with “oba-san” and “oji-san” respectively. Just don’t hold the vowel out, because then it turns into grandma and grandpa (“obaa-san,” “ojii-san”). It’s kind of how in Hispanic culture any friend of your parents’ is your “tio” or “tia” (uncle or aunt).
While they aren’t used much in a cultural way, but mostly a literal way, here’s some others for ya. Mom = Okaasan, Dad = Otousan (pronounced Oh-tow-san), Little brother = Otouto, Little Sister = Imouto. You might hear different words used when referring to parents, such as Haha, (third person, “MY mother”) Chichi (third person, “MY father”) or just straight up “Mama” and “Papa” which are much more personal like “Mommy” or “Daddy.”