I don't think the correct way is to tell the girl going braless is wrong.
But it should be more of creating a home where the girl understands and respects where the boundaries are and at the same time how she can trust her own judgement when to and not to wear a bra.
Not wearing a bra doesn't mean she has "slutty" intentions but rather that she trusts the people in the home enough to feel comfortable.
Lets says if the concern is because she has a male sibling and your concern is that the brother is rather bothered or distracted by her bralessness.
Either you tell the brother to control himself, and understand that after all she is his sister and understand what a female body is and that he should respect it.
For the girl, as long she feels comfy it is ok. But if lets say you are uncomfortable her remaining braless if there is a male guest. Maybe you should voice out with the mom your concerns.
Again, I don't think the concerns should focus more about her body and that she is cheapening herself. But more of that some guest do have different belief systems and may view your bralessness differently than the family does.
I am not promoting nudity. But in some European families, they usually make it a non-issue for the family to see each other naked. They just see it as everybody's body is just like it is, and that there shouldn't be inappropriate thoughts when seeing a family member naked. I believe this actually forms a closer family bonding and openness that would lead to easier communications within the family.