The question states that there are 15 other girls apart from Maisy.
Therefore, there are a total of 16 girls in the school together with 10 boys.
Total number of children in school = 16 + 10 = 26
Since a family cannot have more than 2 children in the school, a child cannot have more than 1 sibling.
Three girls each have one younger sister. (6 girls)
Two girls each have one younger brother. (2 girls and 2 boys)
Two boys each have two brothers. (4 boys)
The last sentence states that two other boys have sisters in school. That is the vice-versa of the two girls with brothers.
Number of girls that have a sibling = 6 + 2 = 8
Number of boys that have a sibling = 4 + 2 = 6
Total number of children that have a sibling = 8 + 6 = 14
Number of children that do not have a sibling = 26 – 14 = 12
Hence, D is the answer.