If you had your baby boy after your first daughter or you had a baby girl after two boys, you probably have dealt with your son's desires to wear his sisters’ dresses or your daughter wanting to wear her brothers’ clothes. For some parents, these natural behaviors can accumulate to a burdening worry that isn't necessary in most of the cases.
In fact, there is no correlation between children's cross-dressing and having problems identifying with their own gender. It is a perfectly natural desire for children between the ages of two and four, and sometimes it persists well into their fifth year. Although at this age your child can tell the difference between the two genders, they don't yet fully fathom what gender they belong to and what the expectations are. They simply love it when they live their siblings' experience, whether out of curiosity or for the mere love of imitation.
To be fair, if it is perfectly acceptable for a girl to copy her mother and try on her high heels, why should we deny her brother the desire to try his sister's beautiful dresses? To further relieve your worries, this stage in their life won't last long when you understand its causes and tackle them properly.
Don't sweat over your child's desire to assume the role of the other gender out of curiosity, but the problem might get undesirable dimensions if they receive negative comments the like of "girls are more beautiful than boys" or "boys are stronger and better than girls". The child might also notice that you spend more time choosing his sister's outfits, or that you bring her a beautiful bike while you settle for an uninteresting ball for him. In this case, the child likes to change his gender in an attempt to gain your attention and escape the state of negligence you have unintentionally put him into.
At this stage, you need to understand your child's behaviors, where they come from, and what they indicate so you can explain to him in a simple way that it is natural to have a gender different than his sister. Use the difference between his mother and father as a good example. Be careful not to show more love and compassion to one over the other, try to clarify to them the privileges of their gender, and find them friends of their kind.
As for when you choose the toys, try to bring them similar ones. It wouldn't be a problem if you bring your son a doll and assign him the task of taking care of it. Teach your daughter to play soccer and ride the bike, and allow them to have shared interests. Buy accessories for your boy when you are out shopping for beautiful hair bands for his sister so he won't feel he is being left out, and buy both of them sunglasses so they could find more similarities than differences between each other.
However, if this problem persisted after your child gets into school, consult a psychologist to help you study the real causes of his or her behaviors and protect them against any potential effects.
On the other hand, don't raise your sons and daughters like they are one individual with identical needs and desires. It is normal that you find differences between their personalities that require you to adopt different educational methods and adapt them for their best benefits. For example, your boy might lean towards being adventurous, and that doesn't mean your daughter has to follow suit. In the same vein, your daughter's quiet character doesn't entitle you to scold her brother because he is energetic.