What is the difference between younger and youngest?

"Younger" is the comparative form (used to compare one thing with another without regard to other members of the set), while "youngest" is the superlative form (used to state absolute position among all members of the set).

Assuming that the birth order of these children was: boy/boy/girl, the boy in the middle may be referred to as the firstborn's younger brother, since he is younger than the firstborn. His sister is his youngest sibling, but his younger sister. Since she is the lone female child, there is no need for the superlative form.

Suppose, however, that there are more siblings, and their birth order goes like this: boy, girl, boy, boy, girl. Now the eldest brother has two younger brothers and two younger sisters, but only one youngest brother, who is still not the youngest sibling. That title belongs to the baby girl.