Brother / Older sister, also used to address strangers

Literally

Brother / Older sister

When to use it

Bhai (brother) and apu (older sister, informal form of apa) are used constantly to politely address people close to your own age who aren't relatives, shopkeepers, waiters, classmates, strangers on the street.

How it feels

Friendly and respectful without being overly formal; the default way to get someone's attention politely.

Formality

Neutral, safe for strangers and family alike