With regard to having anything to do with a wedding with possible Wiccan influence, one biblical issue I would use to consider this situation is the "food offered to idols" issue. Early Christians were divided on this. Some wanted nothing to do with meat originating from sacrifices in pagan rituals and would not eat it. Others thought that the idols were just a meaningless hunk of wood or rock that did nothing to the meat. Paul's response was one both of conscience and accommodation. If it doesn't bother you, go ahead and eat it. If it does bother you, don't eat it. The main issue being that if you think it is okay to eat it, make sure that you are not encouraging someone (who thinks it is wrong) to violate their conscience by copying you.
I'm not a drinker and rarely go to bars. I worked at a place where the going away parties for people were held at a local bar. For a couple years, I never went. Then I realized that it was a nice show of support for the person leaving. I would attend (and walk around with a can of soda). Many people who the parties were for appreciated my presence there (since most people knew I didn't drink). Ultimately, the big thing for me was not about taking a moral stand against drinking but rather maintaining relationships and doing things for people. Here's reality, me walking around with a can of soda was probably not causing anyone to take up drinking and it was a way of establishing stronger relationships with people. Perhaps I also showed a few people drinking that it was possible to hang out like that and not drink.
The main things I see are these. Will there be anyone there who sees your participation as actively encouraging them to become Wiccan? Is this something that your brother and his wife will deeply appreciate knowing that you have differences on it? Will it open the doors to ministering to them in some way? Will you be doing something that violates your conscience? Ten years from now, how do you think you would view your decision? What is the main thing that God is putting on your heart to do? (These are just food for thought. I'm not expecting answers.
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If you feel like God is putting it on your heart to be a bridesmaid, go for it. God might be putting you into a position to show His love and shine His light into the lives of your brother and his bride (and perhaps some of her friends). Who knows, you might have an opportunity to get into some discussions of spiritual matters with a few people and have the opportunity to pray for them or something. If you feel like God is convicting you and warning you not to be a bridesmaid and stay clear of things, don't do it.
The only caveat I'd give is that if you decide to be a bridesmaid, you might want to consider ahead of time how you will respond if asked to do something "Wiccan" during the ceremony and ask for God's guidance and grace to respond as He wants you to.
