。
At work she's known as 'auntie.'
Literal
She [topic-は] company [at-では] auntie [as-で] is-going-by.
通る has a wide range of meanings ('go through, pass, run between'), but here it's the figurative sense 'to go by (a name), to be known as.' The で attached to おばさん is the で of role or status — like 'as auntie' — and 通っている (~ている for resulting/habitual state) tells us this is her settled informal label at work. おばさん, literally 'aunt,' is also the colloquial address for any middle-aged woman, often warm but sometimes unwelcome — younger women generally don't enjoy being called this. The では (= で + は) is a contrastive で: 'at the company at least' (with the unspoken 'though perhaps not elsewhere').