First of all, comparing her own exclusion to the absence of the groom's late mother ... there are no words. Secondly, I'd have a lot more sympathy for Sarah if she hadn't made her own bed, with a long, long string of bad decisions and less-than-ideal behaviour. Thirdly, I, too, invited an aunt and her child, but not the now-divorced father of the child, to my wedding last year. I think that's pretty standard.
I did see part of the interview where she said that it was bittersweet for her because she was the last bride down that aisle, and I can understand that feeling, and am even ok with her expressing it. But the only way I could have seen her being invited was if the girls were still young, and were in the wedding party. As it is, I can't imagine who would have thought she would be invited, and it was hardly a snub.
I thought when the invitations first went out she handled it well, issuing a statement about how she didn't expect to be invited and she wished them well. This pity party after the fact, and trying to ride the wave of affection and remembrance of Diana, is a bit much.