Does she HAVE TO know what your mother gets?

Personally, I think its best to give your father's wife a gift too...maybe something nice, but not as nice as what you'd give to your mom. Nice gifts have the power to mend hard feelings and heal peoples' hearts. You never know...maybe giving her a nice gift will change her attitude towards you for the good. You're entering a marriage, the happiest time of your life, and I'm sure you wouldn't want any stress or problems in the family during this time. You'd want things to be happy for everyone, for your own sake. If you don't want to give her a gift for HER, give her a gift for yourself...for your own happiness. You will be the better person because of it, and it could possibly result in changing your relationship for the best, in the end.

Good luck on whatever you decide, and best wishes for a happy, lifelong, committed marriage!