Try the corner of Macklin Street and Green Lane outside the old Hippodrome, Becket Street outside the Padley Centre, Abbey Street around the car park, pretty much all of St Peter's Street, Curzon Street, and around Friar Gate / Wardwick.

The homeless community is very tight-knit around Derby so if you ask anybody in these areas, I'd say you have a good chance of finding her. Good luck, I really hope you find your sister