My father put his second wife's name on the same headstone as he and my mother?!?

I think he could have done it differently. If your father had kids with his second wife i would understand a little, but then he could always put one with his 2nd wife's name right by his and your moms. Or maybe each of their names in separate headstones. But think about your Dad as well, maybe these were two of the most important episodes of his life and he feels like he would like to cherish that forever. If i were in your mom's shoes i would not be mad, everyone has a right to fall in love again and who says it can't be as strong if not stronger that the first time. However, if there were no children with your step mom then that is bull, because no other relationship, no matter how much love and connection can compare to that of having something as meaningful as children.