Obviously you are watching a soup opera TV series from Taiwan, I will try to explain it to you from my perspective.
"三八" is used to refer to women because 8th March is the Internional Women's Day. Since you mentioned that these two women know each other so well, I'd say they are friends. You know when you are good friends like Max and Caroline (《Two broke girls》) , you get to say mean, vulgar, derogatory words all the time. Because that's what friends do, they joke a lot as they have a much closer thing than with most other ramdon people.
Here you can't translate “三八啦” as the normal way, gotta add some scenario like "bitch, just shut up and eat before I change my mind." without actually thinking about changing the mind.
Hope my answer is clear enough for you.