The first thing I'd like to say here is tread carefully! Emotions always run high during the build up to weddings and things can easily spiral out of control. I can completely understand your feelings regarding the role your daughter's stepmother may play in the wedding. However, I do believe that on this important occasion, it might be wise to just grin and bear it for your daughter's sake. It sounds as if she's being pulled in so many directions and is desperately trying to keep everyone happy with regards to their involvement. I'm sure she knows how you feel about her stepmother but I imagine you daughter is hoping you can be swayed more by your love for her rather than your dislike for her stepmother. I also believe that saying no to this request will rebound badly on you, despite your reasons. People will notice that you're not standing there so I would try to quash your feelings for the few moments of the unity ceremony and focus on how much it would mean to your daughter. In my personal opinion, attaching conditions to your involvement with the unity candle doesn't bode well. I'm sure everyone at the wedding knows how important you and your husband are to your daughter, you have nothing to prove to the guests but you have everything to lose in your relationship with her. In a situation where so many people are creating issues for her, I'm sure she'd appreciate your unconditional support more than anything else.

I feel that this is an issue to be discussed and sorted out by the squabbling parents, not the daughter. If the daughter had any strong feelings then I feel sure that she would have expressed them so it should be up to them to come up with a solution. If one parent from each couple has an important role in the day then that should even things out. For example settle for the biological father to walk her down the aisle if the biological mother lights the candle, then perhaps the step parents could be witnesses. Hope this helps.