Dear Prudence,
My cousin "Bill" is getting married next month to his fiancee, "Jane." They live out of state and have had a short courtship, so most of my family hasn't met Jane.
After they set their wedding date, my older sister "Tammy," who has not met Jane, sent Jane a message on Facebook introducing herself and asking Jane to change her wedding date because Tammy had a conflict. Jane declined, there were subsequent hurt feelings, and Jane decided not to invite my sister to the wedding.
This has caused a problem in my family, as other people are now refusing to go to the wedding because my sister is not invited. I thought what my sister did was selfish and rude, but also that Jane's response was a little much. I talked to my sister about apologizing to Jane to smooth things over, and she refuses. She says she didn't do anything wrong and Jane is crazy for being upset.
My concern here is that this is going to be a feud that will last for years. Can you please help my sister understand why what she did was unacceptable and that Jane's response is not crazy?
—In the Middle