An astute (oblivious) Rainbow Dash spends her days working, lounging, and hanging out with her best, completely platonic, in no way pining, 20% tsundere sis, Anon.
This story takes place in an AU called RGRE (Reversed Gender Role Equestria). If you're unsure of what that means, there's a group on fimfic that catalogues stories like these that might explain it best. If you're interested in content with a similar setting, consider joining it and looking around.
Cover Art by Affanita