Antonio married Beatriz. After a time of happy marriage, they had a baby. Years later, that baby grew up, married his partner and they had a daughter named Cristina.
The last names of the 5 family members mentioned are, in alphabetical order:
What is the sex of Antonio and Beatriz's son?
We can try to recompose the family tree to determine the sex of the baby but the easiest way to solve the puzzle is to notice that there are not three people whose first name is the same, so the descendant of Antonio and Beatriz must be a girl. If he were a child, Antonio, Antonio's son and Cristina, (Antonio's granddaughter) would share the same first name.