"Your sister is prettier than her."
Translation:Tu hermana es más bonita que ella.
Exactly. However, you hear "...than her" coming from the mouths of native speakers all over the place. People are literally ignorant of what was formerly considered correct usage. Ah well. On one level no great loss. On another, ugh.
Que is used to compare two separate things or instances, whereas de is used when comparing a thing against a set quantity. This article might help: http://spanish.about.com/od/sentencestructure/a/mas_que_de.htm