Thai addresses people by relative age, using kinship terms, whether or not there is any kinship. พี่ is an older sibling and น้อง a younger one, and both are used constantly between strangers.
The waitress calling you พี่ is not being familiar. She is placing you as the older of the two, which is the ordinary courtesy, and it happens before anything else in the conversation.
- พี่pʰîː
older sibling; you, if you are the elder
also how you address the person serving you, if they look older
- น้องnɔ́ːŋ
younger sibling; a younger stranger
how you would address a younger server
- ลุงluŋ
uncle, a man of your father's generation
- ป้าpâː
aunt, a woman of your mother's generation
- น้าnáː
younger aunt or uncle, mother's younger sibling
Why it is worth the trouble
These words replace 'you' and 'I' far more often than คุณ or ผม do. Using someone's age-term instead of คุณ is warmer and much more natural, and it is one of the fastest ways to stop sounding like a textbook.
Guessing wrong is not a disaster. Erring older is the safer mistake: calling someone พี่ who turns out to be younger flatters them, while the reverse does not.