TLDR at the end. My family (my husband, my mother, our son, his girlfriend, and I) were just on vacation together in a pretty fancy location. We're taking a vacation again together later this summer with similar vibes. We are in the US.
My son's girlfriend is lovely and we really like her and having her on trips with us or just around in general. The issue is that she wears some shorts that don't cover her behind at all; half of her rear end is on display (her bikini bottoms cover more than the shorts do), even when we're shopping in town, and not just hanging out at our rental house. She also wears some tiny cropped tops with spaghetti straps without a bra in public with us.
My mother said that she'll be embarrassed if she dresses similarly on our next vacation, and I will be, too. The problem is that I don't know to handle this without coming across as a total witch, or like I'm body shaming her – something that is completely taboo for their generation and not at all my intention. I just worry that it's going to cause friction with my mother, and that it'll be embarrassing for the rest of us. I'm not against crop tops or spaghetti straps or short shorts (I wear these sometimes), but I wish they covered more than hers do.
Any advice on how to address this would be much appreciated! I don't want to hurt anybody's feelings! Thanks!
TLDR: How to address son's gf's habit of wearing very revealing clothes when we're in public together?
UPDATE:
I appreciate those of you who provided thoughtful feedback. Of course, I can't respond to everyone, since this blew up. I've decided to say nothing to my son or his girlfriend, but to ask my mother to try to understand that if his GF is comfortable wearing something, we need to accept it. Before we leave, I'll take the "kids" shopping for some things to wear on our trip, as some have suggested. Thanks!