But let's go over some of the psychological facts that we do know. Most children at the age of two are starting to have some form of gender identification (meaning that a girl knows that she's a girl and a boy knows he's a boy, and knows that there's a difference between the two), but not all. About 90% of three year olds will certainly have this gender identification. Even with it, however, there is no specific reason why your child needs to be protected from seeing these gender differences, and indeed it is an important part of development for children to understand these differences. It is not until ages 4-6 that children develop what is called gender consistency and gender stability, meaning that a girl knows she will grow up to be a woman, not a man; and that even if a boy wears long hair or a skirt that he is still a boy. It is also at about this age that many children start to have some sense of modesty about being undressed in front of the opposite sex (though this is likely more learned than inherent). By age 7-9 the concept of privacy is usually well established, though determining what needs to be private is largely due to family and community teachings.
So you should make your decision about when to stop the joint showers based on your own level of comfort, along with some understanding of your child's development and level of comfort. You will not cause any psychological harm by showering with her, unless you ignore her wishes when she tells you she wants to stop.