Personally, I would tell her that if she does not want to go and witness her son getting married, fine, you will send her pictures :-) she will feel stupid for having let fear miss her son's wedding while everyone else enjoyed it...like your sister said it is your wedding, you are marrying the man of your dreams and this is the most important.
She is probably hoping you will change your mind and have the wedding at home....don't do it, otherwise, she will rule your life.