I was called "sister", as a replacement for my name. (Oddly, my brother was not called "brother.") I never questioned this growing up in the 50's in a rural area. It says much about the culture I grew up in. Yet I cannot explain it well, to younger people or to those not from a rural upbringing, in the 50's and prior.
This was reflective of my role in the family, not any sisterhood. A friend was confused by this, only understanding sister as in sisterhood.
I haven't been able to find any background or etymology on any website. It's simple but I'm interested in cultural aspects as well as any fiction reflecting this.