Here is the simple explanation:-

i) we understand Mr. Robert has four daughter ===> 4

ii) and we know that each of the daughters have one brother ==> 1

iii) so total no. of children ==> 5

got shocked Or confused ??… many people thought like four daughters have one different brother so 4*2=8 (Eight children)

if you read this riddle in grammatically way, all of the sisters have one common brother so 4+1=5 children