The answer is 1/3

Alright, Let me try to Solve it.

So, you said Couple has 2 kids and one of them is girl. So, The combination of having 2 kids (Boy and Girl) would be :

Girl - Girl
Girl--Boy
Boy-Girl
Boy-Boy

Since one of them is already a Girl which leads us to elimination of Boy-Boy combination. Therefore, the probability that the other is a girl to is 1/3 .