Let the age of Kevin be x,
age of twin sisters = x - 6
sum of children's age = x + x - 6 + x - 6
= 3x - 12
3x - 12 = 42
3x = 54
x = 54/3
= 18
age of twin sisters = x - 6
= 18 - 6
= 12
Kevin is 18 years old whereas the twin sisters are both 12 years old.