from the first statement, it implies that Dominic is 8 years older than his sister.
first we represent unknowns using variables:
let x = sister's age
let x+8 = Dominic's age
since the sum of their ages is 56,
x + (x + 8) = 56
then we solve for x:
2x + 8 = 56
2x = 48
x = 24 (sister's age)
x+8 = 32 (Dominic's age)

Well, if Dominic was 8 years old when his sister was born, and now the sum of their ages is 56, I'd say Dominic is probably starting to feel a little older. But, to answer your question, Dominic is currently 28 years old!