Yeahhhh, I mean, this is a LEGALLY responsible adult, but this isn't a person who has the life experience and the perspective of an adult, nor should you expect them to be as responsible as any other adult. Her sister is actually still a teenager! You can't be surprised by a 19-year-old agreeing with the 16-year-old; the surprising part is that she had the balls to pretend to be the 16-year-old's legal guardian and that the tattoo parlor bought it. I'm sure in 90% of cases this gambit would not work, and in most of them it wouldn't even occur to the kids to try.

But ultimately I feel like her anger is disproportionate given that her child is so close to being legal anyway. Kids at that age are very close to independence and it's not uncommon for them to take stabs at it - if their parents stand in their way, they just do it behind their backs. Certainly as far as those rebellions go, this is a pretty mild one with minimal harm to anyone.