So after Sunset returned to Equestria, she lived with Twilight initially as a guest. But over time they started spending time together, nerding out over science-y things, then taking slow walks, cuddling on park benches and just being glued to each other.
 
Starlight did not like Sunset. She just shows up out of nowhere, she’s an alicorn and a former student of Twilight’s. Starlight was jealous. She had a severe crush on Twilight and felt Sunset was slowly replacing her. She started stalking them on excursions, showing up uninvited sometimes and trying to one-up Sunset with her magic and friendship expertise. Sunset is aware of the jealousy but isn’t into drama and Twilight is oblivious. Starlight refuses to lose Twilight, and a violent confrontation occurs.