^it's essentially just the basic old teen drama of jealousy over a friend finding new social circles and drifting away from their old ones, what now most of the central figures are toxic jerks. We do know from earlier special-chap vignettes that Sachi is quite fond of Minami (in as warped a way as you'd expect in that clique) and the flashback in recent chapters indicates they go a fair way back (Yuri and Minami used to at least tolerate each other's presence back then too apparently, nowadays they actively avoid direct interaction).
Sachi's probably rather more gay foremotionally invested in Lil' Fang Bitch than she realises or would ever admit, and seems to be lowkey panicking on belatedly awakening to the fact Minami's slipped her moorings and discovered new harbors (so to speak).
Her reaction to the whole thing is about as mature and healthy as you'd expect from the general psychological profile of the group - not that someone like her was likely to even know how to act different anyway. She's probably already too late in any case and essentially forcing a binary choice on Minami will likely blow up in her face pretty bad, but that's probably not going to make Fang-chan's character developement arc any less emotionally taxing for many of the people involved.