If the sister has her own wealth, the brothers are not obliged to provide for her.
If she is in need, this becomes mandatory upon them.

in islam who is financially responsible for the maintanance of an unmarried female whos closest male relatives are her brothers (heirs). Males receive a 2:1 share of their parents inheritance. As they are protectors and maintainers of women. Does this verse cover all females or only apply to wives and secondly does it exclude females who work? Be that a daughter or sister?