I'll take your word for it, but I thought she was describing her as a big sister informally (as in a woman slightly older than them) and as an employee separately. Otherwise, who's the employee that she's a big sister to?
You've got a point. Hikari and Yuina have already met Mai a couple of times before, but it's only in this chapter that we learn what her name is. It doesn't seem like they've ever met Mai's sister before, so having Yuina say, "It's the employee's big sister" or "It's the store clerk's big sister" wouldn't make sense.
Let's rewrite Yuina's first sentence in romaji to make it easier for non-Japanese-speaking people to read:
"tte tenin no onee-san jan!"
The word "tenin" means employee, clerk, sales staff, shop assistant, and the like. As restaurants are also considered shops in Japan, you can call waitstaff "tenin-san."
The word "no" is a particle that may indicate possession or location. So "tenin no onee-san" would mean "the store clerk's big sister" in the context of this manga chapter.
The word "jan" is a very casual word that young people would use with each other. At the end of a sentence, it carries the connotation of "right?" or "don't you remember?". Using that word with older people or in a formal context would be considered very rude and disrespectful.
The word "tte" is a particle that is used in quoting what someone else had said. It's a casual word that is never used in a formal context.
So "tte tenin no onee-san jan!" can be translated as "So you're saying that she's the store clerk's big sister, right?"
Which still leaves me confused in the end as even that doesn't make sense to me.
Maybe someone else can shed some light on this.