There's something especially disgusting about immodesty in MOTHERS. Perhaps because one assumes that a mother is more mature, and is about to guide and educate a new young life, so one hopes that a mother will have more virtue than, say, a young single woman.
A mother generally has had longer to grow in virtue, all things being equal. And there are some experiences that should make one grow up a certain amount "overnight" as it were.
Just like many sins are more disgraceful in the elderly than they would be in a 15-year-old.
Which is more disgraceful? An 18-year-old speeding on a motorcycle (30 MPH over the speed limit) or a 65 year old doing the same thing? A 10 year old shoplifting, or a 50-year-old shoplifting?