Vector taken from S3 E12. This is from a flashback of Rainbow Dash. My guess is that he must be her father. But for all we know, he could be her uncle or older brother, etc.
She is actually just a bg pony that appears often, and is most likely unrelated, and she has interacted with rainbow dash before, without any actual acknowledgement, as well as her voice being to young sounding.
Hmm...maybe Sonic Dash?
Well, that leaves Fluttershy to yet reveal any of her family.