smc

The age of each person is a factor of $128=2^7$. So the twins could be $2^0=1,2^1=2,2^2=4,2^3=8$ years of age and, consequently Kiana could be $128$, $32$, $8$, or $2$ years old, respectively. Because Kiana is younger than her brothers, she must be $2$ years old. So the sum of their ages is $2+8+8=\boxed{18}$. The answer is $(\mathrm{D})$.