Advice

I am getting married this February and have had my date/venue booked for a little over a month, my fiancé’s cousin is also engaged but is just starting to plan his wedding. We have a really good relationship with his cousin and his soon to be wife, we talk wedding plans often, and today they were over for a bbq and she informed me she was thinking about having her ceremony first and then a reception 6+ months later, the date she would want for the ceremony is 3 days before my wedding. I am really bothered by this and I know it will cause drama, but don’t want to say anything yet before It’s a set plan. I really don’t think I am overreacting, I would never do this to someone I’m even slightly close with. I really want to hear other opinions and how you would handle it! Thank you!!!