The question in the application form can definitely be confusing, but the meaning becomes clear when you break it down. When they ask “In case of girl candidate, are you the only girl child of your parents?” , they are not asking whether you are the only child in the family. They simply want to know whether your parents have only one daughter .
In your case, you mentioned that you are the only girl child of your parents , and you also have a younger brother. Having a brother does not change your answer, because the form is asking only about the number of daughters , not about the total number of children. So even though you are not the only child, you are the only daughter , and that fits the category.
Therefore, the correct answer in your application should be YES .
This question is generally asked because some institutions have special fee concessions or scholarships for the only girl child category. So it’s important to choose the right option to ensure you do not miss any benefits.
If you feel confused about any other part of the application form, you can ask anytime. All the best.