Girls Clothing Limits in summer.
It's a wonder your father wasn't arrested for your family going naked when guests were visiting. They may have been OK, even enthusiastic, but if their parents had gotten wind of the goings-on there would have been Trouble!!
If you wear a short skirt, like halfway up to your crotch from your knee, wear a slip or underskirt that is more full so when you sit you don't have to be so careful to keep your knees together to prevent upskirt peeking.
Your parents creep me out. Stop inviting, or allowing them to come to your home. Tell them to stay in a motel and you will meet them at a restaurant. If you DO let them come over, let them know in no uncertain terms that you are an adult now, not their "little girl" anymore. If you can't bring yourself to stand up for yourself, hide your clothes they don't like in your car or somewhere they don't have access to, like at a friend's who already knows about your parents' "quirks."
Bull$%!@, you PERV. It's her parents who need to be punished.
Go away you MALE PERV. Go whack off until it's raw and bleeding and maybe you'll get some relief.