The probability is 1/2.

Here's the reasoning behind this:

1. We know that there are three girls and two boys in the family.
2. The oldest sibling is a girl, so we have already accounted for one of the girls.
3. Now, there are four remaining siblings, with two girls and two boys left.
4. The youngest sibling could be either a girl or a boy, and since there are equal numbers of both genders among the remaining siblings, the probability of the youngest sibling being a girl is 1/2 or 50%.
In a family of five siblings, three are girls and two are boys. The oldest sibling is a girl. What is the probability that the youngest sibling is also a girl?