Let Bill be x years old,let Caroline be y years old.
Now,
"Bill is 1 year less than twice as old as his sister Caroline."
twice as old as his sister Caroline means (2y)
Bill is 1 year less than twice as old as his sister means (2y-1)
(2y-1) is Bill's age,which is 'x'
x=2y-1
"The total of their ages is 26"
that means
x+y=26
we know x=2y-1,so put this value in the above equation
x+y=26
(2y-1)+y=26
3y-1=26
3y=26+1
3y=27
y=9
x=2y-1=2(9)-1=18-1=17
Bill:17 years old,Caroline:9 years old
Hope this helps,
Prabhat