A story in WriterMick's the The Children's Table Universe
The family flew to Charleston so my parents and my Uncle and Aunt could take a cruise for Labor Day. They got a big suite to share but doubted they would leave the suite and turn the weekend into a fuckfest. My sister and our twin cousins thought the same thing as we stayed in the house while the parents were gone.
Jill comes home to find her brother and her cousins having sex. When sneaking into her room she discovers their parent having swinging sex and acting out fantasies. She plans a birthday party for her dad where he takes her virginity thinking she is someone else.
I'm sure my sister is no different to anybody else's. She's reasonably happy to wander about the house in her bra and knickers, I suppose she doesn't really see me as being male for I am two years younger than she is. On a couple of occasions I have even caught her walking from the bathroom to her room wearing just a large bath towel around her waist and a smaller one around her wet hair