In families of all male children (or where the only daughter was too young to assist with the 'women's' work), parents would often choose one or more of their sons to help the mother. Because these boys would perform tasks that were strictly the work of women they were raised as if they were female. Although their true gender was widely known, they would usually be dressed as girls.
As they grew older, their duties would not change. They would continue performing 'women's' work, even if they eventually married (which would be to a woman).
What's interesting about this cultural characteristic is that many cultures throughout the world are male-dominated, and women are treated more as objects or servants, but in Samoa, being a female is just another option.