Is that ok for younger sister to wear?

I was out with my 19 year old gf andher 17 year old sister. We took her to a very fancy place to celebrate graduation. Exclusive restaurant.We were all dressed up. She wore a very prettybut short black dress. I noticed she was adjusting something during the nite. And men at the bar were checking her out at our table. Shes gorgeous and tall and does look older. Thenshe says 'ugh my garter strap is driving me nuts'.My gf says 'ur wearing a garterbelt?' She said yeah to keep her thigh highs in place. My gf andi were puzzled. Is she too young to be wearing a garterbelt and stockings underher dress? Just seemed like an adult item. Let us knowifwe should say somerhing.