Actually, yes, I know of the following girls:

1. As a teenager, I knew a girl who complained that her younger sister had been making repeated, blatant sexual advances toward her.  It went on for some time.  At first, the sister would respond to the refusal with, "ha ha, I was just kidding, you thought I was serious, didn't you!".  As it continued, though, it turned into, "C'mon, why not?  Pleeeaase?"  I never heard how it stopped.

2. I once dated a bisexual girl who had been sleeping with her aunt for years.  The aunt (by marriage) was just a few years older than her.  Once she told me about it, it was pretty obvious.  I'm surprised her uncle never figured it out.  I suspected she had also slept with a male first cousin at least once.

3. I knew a girl in college who was essentially straight, but every year while growing up, during her family's annual holiday get-togethers, she would sneak off with her female first cousin and have sex.  She told this story to a group of people at a party, me included, while very drunk.  We named a veritable laundry list of possible sexual acts they could have done, and she said they had done just about all of them, except toys.

4. An older friend of mine knew of a girl living in a trailer park whose older sister talked her into performing oral sex on all of the older sister's female friends; before long the older sister got her going down on her, too.  This girl had no other friends to compare lives to, and had been doing this so often, for so long, that she thought that this was totally what girls did together.  Once she started high school and made new friends, she quickly realized the truth and stopped.

5. The main one I know of was a girl who, as a young teenager, was coerced into having sex with her older sister on several occasions.  The last time, a family member walked in on them, and the older sister was placed in a psych ward for a while (early 1980s, I think).   Later, she quite willingly began having sex with her older brother.  I mainly knew her best friend, who had a threesome with them, and admitted that she really enjoyed watching a brother & sister fooling around.  The girl kept sleeping with her brother until she had a pregnancy scare, and put a stop to it. 

There you go.