Before I set my wedding date, I checked with my potential brides maids, MOH etc... to make sure that they would be available on that particular date and that they would be able to take time off from work. Once I confirmed that everyone was available, I booked. I wanted to make sure that the people who are important to me would be able to stand by me on my BIG day. With that said.... I agree with everyone else, just be happy for her and go to the celebratory dinner. Since you already have another wedding to attend on the same day as hers, which she's aware of, don't change your plans. I had a similar but different scenario. My cousin was asked to be a bridesmaid in one of my girlfriend's (Lisa) wedding. Another friend of ours (Maria) changed her wedding date to the same date as Lisa's. Because my cousin and I already had plans to attend Lisa's wedding, Maria couldn't be upset with us. So we declined Maria's invite and attended Lisa's wedding.