Awkward parental situation...advice, please?

So my FH's mother passed away 5 years ago. We are doing a memorial chair at the ceremony for her and there will be a memorial table at the reception. FH's father is remarried to a very obnoxious woman that we are polite to because that is his father's wife, but we don't have the warm fuzzy relationship. She is not directly involved in any of the wedding planning or anything like that. I was planning on doing something special (not sure what yet...might just be flowers or something given before the start of the ceremony) for my mom because of all the help she is giving us for the wedding (financially and making my cake and helping with all my other DIY adventures...). We are also giving my parents a nice gift a ways ahead of the wedding for all of their support but that will be given privately. Is it awkward/rude to only give my mom something at the wedding and not FH's stepmonster? I feel that the little something we are wanting to do at the ceremony is just a little public thank you for all she's done for us but I don't want to blatantly be disrespectful to FH's dad's wife. However, I also don't want to give the stepmonster any sort of recognition that she doesn't deserve.