Usually if I see doses that high a psychiatrist is trying to treat obsessive-compulsive disorder (OCD), though you didn't mention that your sister had it. Unnecessarily high doses expose patients to higher cost, more frequent side effects, withdrawal from missed doses, potential interactions etc. Prozac is pretty potent stuff and I've never had a reason to push it past 60mg a day expect in one or two cases of OCD. However at 100mg Prozac starts to generate more anxiety than most people can stand. In general I very, very rarely have to exceed the published dose ranges with antidepressants. If you take a careful history on people's previous medicines and intolerable side effects you can usually pick one they'll tolerate right off the bat.