let

x = the age of Larry now

y = the age of his sister now

Larry is 8 years older than his sister:

x = y + 8

In 3 years, he will be twice as old as she will be then:

x + 3 = 2(y + 3)

by solving the system of equations:

x = y + 8

x + 3 = 2(y + 3)

we find:

x = 13 years

y = 5 years

click here to see the step by step solution of the system of equations:

Larry is 13 years old now and his sister is 5 years old now.