My FI's cousin invited her son's girlfriend (they are 20 years old) to our wedding. I have never met her son and FI hasn't seen him since he was a little kid. We have never met the girlfriend. Part of me wants to just let it slide to avoid awardness/confrontation but it really bothers me that she added her to the RSVP. The invite was specifically address to the her and her husband and their 2 kids. Ugh, so rude! Our guest list is already larger than I wanted it to be because of large families and our friends that we want to be there...it does not include guests for the guests! What would you do?