If your family is trying to adhere to the Traditional Rules of Etiquette, then, yes: showers are only thrown for the first-time mother, to help her "stock up" her layette and prepare for her first baby. Any additional children are "gifted" by friends and family after they are born (usually during their visit to meet the baby).
That being said, the Traditional Rules of Etiquette also dictate the following. And we already know that your family is breaking rule #1 (and probably rule #2!!)
1) Traditional etiquette dictates that it is inappropriate for a family member to throw the baby shower. Baby showers should be hosted by friends of the Mom-To-Be. (This rule doesn't seem to be upheld by many people these days!)
2) Traditional etiquette dictates that it is inappropriate to register for gifts. It is simply rude to direct your guests to a list of gifts that they should bring. (Everyone I know, including myself, breaks this rule!!)
But you asked for suggestions on how to handle your situation. I suggest that, instead of being angry on the inside and letting it eat you up, you ask your stepmom why she never threw you a shower for your firstborn. But be prepared for any answer she might give.
Furthermore, she most likely is in alignment with your father about showers only for the first baby only, so she is not going to throw you a shower now. (And, as a first-time Mom, your stepsister likely has no interest in sharing her shower with you).
At least, you can tell her that you still feel hurt that she didn't throw you a shower. I hope that alone can help you to heal. I wouldn't count on getting anything more from her.
I'm sorry you are hurting. Try to focus on the joy that your newborn will bring you soon!