I wouldn’t make too big a deal of someone’s moment of weakness in wanting to have some fun while thinking himself/herself a bit oppressed by wet blanket relatives. Perhaps this is similar to some things that fathers and sons would be doing without telling the mother — for example involving some variety of combat or risk of physical injury or something exhausting and challenging like survival sklls. Perhaps to them the piercings were like just getting a haircut, and, while I can’t be sure, I don’t think we’d be making much of a fuss of them just getting a quirky haircut.
However, this said, I can’t disagree with EasterJoy (this time, of course


) or with Kelfa. There
may be some sort of trouble on that front, i.e. disconnect with you, disregard for you etc.
On the other hand, it’s possible they thought you wouldn’t make a big deal of it, which isn’t exactly respectful but is not the same as just going ahead and hurting you emotionally with full knowledge and intent.
Also, like Kelfa said, there may be a problem with drawing the attention to the body. An interest in one’s body is a natural thing in children and even teenagers, and some kind of a pride in it and desire to experience some sort of appreciation on its account (desire to please, visually, experience another’s interest and appreciation, approval) is natural in women of pretty much all ages. Just simply putting your foot down and thumping against it could have a detrimental effect. You’d need to approach that kind of thing in a delicate and smart manner, with a lot of tact and respect for individual autonomy. It doesn’t take a sexual context for the ‘victim’ to feel violated in one’s body. For example we don’t feel great about being patted or strip-searched by male cops, do we? A young woman could feel violated in having her body ‘regulated’ by an older male, especially over the objections of her mother. Still, you need to pass on God’s teaching, the Church’s teaching, about the meaning and purpose of sexuality. (Though nobody says you have to be gloomy, reproachful and preachy while doing it. St. Thomas More was a great believer in generally having some sense of humour, and I tend to think about him in the circumstances; he too had daughters to deal with, so perhaps seeking his intercession would be a good idea.

)
No envy you. Dread the day I’m gonna end up with a teenage daughter of my own (if).
One woman is more than enough trouble, but four?
