In 2 years Peter will be twice as old as his sister Eve.

p=Peter's age

e=Eve's age

p+e=26

p+2=2(e+2)

from p+e=26 we get p=26-e

substitute

26-e+2=2(e+2)

28-e=2e+4

28-4=2e+e

24=3e

e=8 years old

p=26-8=18 years old

so, Eve, is 8 and Peter is 18

-wolfiechan

in two years : 2e + e = 26 +4

3e = 30

e = 10  in two years    so e = 8 now and p = 18 now