Math …

Let John's age be $J$

Let his sister's age be $S$

$J = 10S$

$J + 8 = 18$

$J = \boxed{10}$

Is something missing here?      in 8 years John will be 18......   so he is 10 now .

Let John's age be $j$

Let John's sister be $s$

$10s = j$

$j + 8 = 18$

John is currently 10 years old right now. His sister is 1 year old.