Brother, sister, son, daughter: trai / gái after the family word

Some family words, like em (younger sibling) and con (child), don't say whether someone is male or female, so trai (male) or gái (female) goes after them: em trai (younger brother), em gái (younger sister), con trai (son), con gái (daughter). Anh already means an older brother and chị an older sister, but anh trai and chị gái are common too, to make clear you mean a real sibling and not just someone you call anh.

The order is the family word first, then trai or gái, like any describing word after a noun. To say whose, add the person after: em gái tôi (my younger sister), con trai chị Lan (Lan's son).

On their own, con trai and con gái also mean "boy" and "girl": Con trai hay con gái? Is it a boy or a girl? The same words make bạn trai (boyfriend) and bạn gái (girlfriend), though they can also just mean a male or female friend; context tells. Brothers and sisters together are anh chị em: Em có mấy anh chị em?

For animals the words are different: đực (male) and cái (female).