They may have enough crazy relatives to deal with, but that's no excuse to not allow most of the brides family members to go...You know if I was in Kate's position I'm so stubborn I'd cancel the damn thing because of that. Disrespectful it is...argggg.
Philip? How on Earth did a post-War Britain take the Princess Elizabeth's marriage to a German whose relatives were members of Hitler's SS. None of his relatives were at the wedding. So it's common for Royalty to completely snub one side of the family at those occasions. Kate Middleton's family have done well in comparison with 15 invites.
William and Kate should do what I did - My ex-wife and I had a quickie wedding at Islington Register Office and told everyone afterwards. Some took umbridge, but, all in all, it saved a lot of problems with invites etc.