So she has two biological parents? In which case she can be certain one is male and other female. Assuming she was not an IVF baby, one has a penis and produces sperm, the other has a vagina and produces eggs. She can make a pretty big assumption about how the eggs and sperm got to meet... especially if her parents are or were in a loving relationship. So she does know about their sex, gender, sexuality (at the time of conception), genitals and hormones. What she should say is that thinking about her parents having sex is a bit Eewww so she prefers not to think about it.