If it really bothered a family member, maybe they should buy the niece or give her vouchers for those stores where the bodysuits come with chest padding. They're really popular among some young adults I know because as others said, going braless in more popular. Otherwise, they can keep their thoughts to themselves and remember what teens were wearing when they were that age.
So you WOULD be okay with your Mum/daughter doing what I described?? Yes??
My mother can do whatever the fuck she wants, I don't care. I saw her - and my father - topless often as a kid. Topless sunbathing was very popular and yes, done in the yard. I thought the Midwestern US is known for its prudishness, but apparently we're out there.
I don't allow any of my children, regardless of sex, to go out topless - but child in the OP isn't topless, she's just wearing a dark bodysuit. Comparing that to being topless or in mesh clothes is nonsense.
I see men in tight lycra/spandex all the time at parkrun and on the footpaths around where I am. We had a guy once in the tightest bright neon shorts I've ever seen going up and down the starting section. I think he scared a couple of the dogs. Even in regular wear, a lot of big guys in t-shirts are going to have visible nipples. A small teenager in a dark bodysuit isn't going to be more noticeable than that unless you're really looking.
It is not about ''body shaming'' to say that if you choose clothing that clearly show the outline of your nipples you are potentially going to attract some unwanted attention and you will not exactly look stylish and classy...
I got far more unwanted attention being covered head to toe with just my face and hands visible than I do now in a tanktop where my nipples outline might be visible if you're really looking. That unwanted attention wasn't my fault just like any unwanted attention this girl gets wouldn't be her fault.
I don't think anyone in a bodysuit is going for classy, nor do I think that's a common teen priority on holiday. Different clothes for different contexts.