How far does a brother's responsibility as guardian (wali) extend toward his working younger sister after their father's death, and until when?

Summary

When the father dies, guardianship over the sister's person passes along the established order, and a full or paternal brother holds it if there is no father or grandfather. A woman's guardianship continues until she marries. The brother should watch over and protect her from harm, for example by not letting her live alone in a place not known to be safe and by ensuring her social interactions are limited and within Islamic rules. He is not obliged to provide maintenance if she works and can support herself, and a guardian has no right to manage the property of someone under his care who has reached puberty.

Written in our own words from the original fatwa — check the source before acting on it.