Smells like drama is on the horizon. I'm like 90% sure this new girl is the same one in his flashback so it'll be interesting to see how that situation went down
I'm pretty sure it is the girl in the flashbacks. Oppa is a term that a younger sister might use for an older brother, but in slang it is commonly used as an affectionate term for women to call their (usually older) boyfriends.
I think "Brother" is a mistranslation of "Oppa". The description clearly stated "first love" and the first flashback clearly indicated she was breaking up with him. Yes "Oppa" is like "Onii-san" in that it does mean "older brother" but just like neighborhood elementary school kids calling the high school boy from their neighborhood "Onii-san," "Oppa" can be used affectionately (typically by younger females towards an older male they're close to or romantically interested in.)
So "brother" is not a correct localization in this case