What is the responsibility of parents and siblings towards a sister who has removed her khimar and worn a tarhah, knowing the characteristics of the legal hijab, and intending to wear the niqab, and what is the way to advise her?

If the "tarhah" (head covering) covers what must be covered, takes the place of the khimar, and fulfills the Islamic requirements, then there is no harm in wearing it. However, if any condition is not met, then it is not permissible to wear it, and she should be advised gently and kindly. If she does not respond, then she should be advised by shunning her if that would benefit her. Her guardian must compel her to cover herself, for she is a trust with him, and he is responsible for her.