All of my fiances 5 sister asked to be in the wedding we are having. About two months ago one of his sisters dropped out of the wedding stating she wanted to wear a dress she had already purchased.
She informed me one month ago that she was inviting six of her friends to the wedding. I told her that was unacceptable.
To get around my protest she had an uncle to send back his RSVP with a response that 9 people were attending. I questioned it when I learned that he was unmarried and had no kids.
Today, I found out that the sister had him to respond 9 to include her friends that I said no to.
Guests are not supposed to add their guests to their invitation, so it is best for your fiance or his mother call this uncle. Plus, it would probably be better for your fiance to talk to his sister. Her behavior is a bit extreme.