Idk if it's the same in japan or not but asian normally call their close friend or family member as the same age as them "bro/sis" and call adult "auntie/uncle"
Rather than brother/sister it's more like "person of ___ gender who I refer to formally"
I dont know about japan but in korea, if you are a female you adress older males usually as 'oppa' and older females as 'unnie.' THe litteral translation of oppa means older brother and the litteral translation of unnie means older sister.
If you are a male i korea, older dudes you usually refer as "hyung" and older females as "noona" which also mean older brother and sister.
You only usually use these terms if they are sorta close in age usually a year - 9 years sorta.
She calls them brother/sister cause they're related(ie cousins)
Oluris and Wilhelm are her cousins, right? She seems to call them brothers, but their mother isn't her mother. I'm confused. (￣∇￣")