Is it permissible for sisters who wish to encourage others to wear the hijab or niqab to mention that they used to love adornment but then gave it up for Allah? And is this considered to be for the sake of benefit (maslaha) or a suspicion of openly declaring a sin?

This does not fall under public disclosure (mujaharah); rather, it is a warning, not mockery. It is disliked for someone afflicted with a sin to inform others about it, unless there is a legitimate Sharia (definition) purpose, such as seeking a solution or a supplication. The proof for this is the story of the man who had intercourse with his wife during Ramadan (definition). However, the interest (maslahah) should be considered, as the general principle is concealment (ستر). If it is possible to invite them by mentioning general stories without explicitly stating the sin, that is preferable.