The Peeters family has four children, the oldest child is a boy and it is known that at least one of the other children is also a boy. What are the chances that the youngest child is a girl?

Answer 1

so actually the answer has to be 3/7; but I don't how.

Answer 2

"What are the chances that the youngest child is a girl ?" is the only real question here

A child being a boy or a girl is an independent event, that is, it does not matter
how many of the other siblings are boys or girls, each has a probability of 1/2
Suppose a family has 10 consecutive boys, the prob that child #11 is another boy is just 1/2

so just plain ol' 1/2