Sunset Dash is one of the many "UC"'s around. She was simply a character for Ponychan's RP Donut Bar. She became a serious character soon after.
Her name was originally ment to deprive her looks (Sunset) and her sister/sister relationship with Rainbow Dash. Later on, the person who made her decided to give her a daughter. Her name being Twinkle Dash, deprived yet again from her looks(She was really a purple/lilac maned foal of Sunset). The roots of the "Dash" family went on, with Sunset's creator going crazy with the whole thing. By now, Rainbow had:
A mum, dad and a gran
Older and Younger Sister
The second daughter was named after her gran: The gran being Sky Dash, and the foal being Skye Dash. She began to build a more steady family root of the Dashie Family, creating a cousin (Skyra Dash) and an aunt, who was never named. In the original donut bar, her "backstory" involved her crashing, and leaving Fluttershy to the responsbilites of raising Twinkle Dash, who was really a newborn at that time. She showed a lot of depression about it, but that was forgotten. She then paired up with a new ship, with a Fluttershy recolour named Blue Waters. This was never explained well enough to make any canonical sense to her canons. That was long forgetten, and her friend had made a pegasi Rarity. Naming her Flawless, this ship showed out nothing but a friendship. A month later, Sunset had came back in full swing with a new friend ; going by the name of Ribbon. Later, Blue Waters' creator made a "twin" for Blue Waters, simply naming her Pink Waters. Sunset Dash, had started shipping before with a Celestia clone by the name of Solestia. This ship never got far, but Sunset had picked up a nickname by then; Sunny.