Huh, I was thinking the guy was her twin brother, but she called the one she talked to sis, so apparently not (unless she has more siblings). To summarize what we know:
The thought in the title is incorrect as proven by her hair being real and the guys hair being short. So it can't be a case of it being a wig with one of the identity's falsifying their sex.
They are unlikely to have a direct familial (sibling) connection (the gf had a sis, and the average household has less than 2 kids).
The guy wants mc to date the classmate (or at the very least someone else than the gf), as shown by him encouraging the classmate.
Also shown by him having contacted the gf about the "big boobs" beforehand (before classmate even asked mc, as shown by the guy not having access to any phone to message with) such that gf ended up texting him directly afterwards
The tags are clear that there are no gender bending shenanigans going on (no fantasy, supernatural, sci fi, gender bender, or TF tags)
I guess the most likely option is that the guy likes the gf, tries to make mc break up with her, and has prolly grown up with her so his interests and mannerisms align (the resemblance suggests a cousin?)
@cursedchocobo he isnt crossdressing, as proven in the hair-chapter. I think it is fairly obvious since
the title the first chapter now that it is going to be (or at least setting up obvious red herrings to make us think so) ranma 1/2, except (since the phone convo) with either split personalities (communicating through texting... somehow), or some kind of time-limited drug/ability sourced by the sister. But since it didn't have those tags at ch1 I have decided to play oblivious and explain it away even once the reveal happens
![Big grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Just for silly fun