Many people actually say they don’t enjoy being a bridesmaid or they can’t afford it but they still want to support the couple. So it’s all the fun without any of the emotional and financial responsibility that is associated with being a bridesmaid. It doesn’t have responsibility of other roles either: finding the perfect reading when that is often the officiant’s job, or something else that may be common.
I was just a guest at my brother's wedding and it was so refreshing. His wife chose a few of my other sisters, who she is genuinely close to, to be involved and there were never any hard feelings. I had fun being able to witness the ceremony with full view from the seats and being able to spend the entire night with my own fiance and family with no other responsibilities.