Is it permissible to apply to charities for assistance with the treatment costs of one's parents and sick brother, especially since the working sisters pay half of their salaries to cover the parents' expenses?

The financial support of needy parents is obligatory upon all children, both sons and daughters, each according to their ability. Whatever financial support the parents require is to be distributed among the children according to their means, and whoever is unable is excused. It is not permissible for you to seek aid from charitable organizations as long as you are able to bear the costs of treatment, because it is forbidden by Islamic law for one who is wealthy to beg.