I'm a bridesmaid in my brother's wedding. Is it appropriate for me to change into a nice suit for the wedding reception?
It's understandable that you might want to change outfits to hang with your family at the reception and not feel like your dress makes you apart of the bride's entourage instead of your brother's. But it's really up to the couple. If it's okay with your brother and his fiance, then that's all that matters -- and so etiquette-wise, it's fine too. But if it's not okay with them, then you should respect their wishes and keep the dress on.
Bridal Party Fashion
Basics for Bridesmaids