We are boys, let’s say something.

I am 20 this year, the same, but if we met, I would not call you sister, I would call you. After all, it means we just met.

But if you are older than me, I will occasionally say something, but it will not be the same as the elder sister.

Girls are afraid of being called old. I know a girl, she is 24. She often says in front of me that she is my sister, older than me, old, and so on.

I like to tell her that we are the same age.

Nowadays, some people who call me sister may not be familiar with them at first, but they don’t know how to say it. Calling me sister is just to be nice and make you have a good impression of her. It doesn’t mean that you will be much older than them, so Well, don't mind. You don't have to respond deliberately when you call me "sister", just treat it as the beginning of a greeting such as "HI".

Sometimes when I have no choice but to call a girl sister, for example, I call you sister, please, etc.

Just ignore this kind of thing, I won’t suffer any loss. If you call it unintentionally, why should the listener pay attention?