You mean, after your step-mother naturalizes, while you're still under 21? It's possible for a citizen step-parent to file an I-130 for a step-child, as long as the child is under the age of 18 at the time the step-parent/step-child relationship was created; but until your status is adjusted you have no protection from deportation and you will accrue unlawful presence if you stay in the U.S. after you turn 18.

