I understand how you feel. Logically it shouldn’t matter that your children, who grew up with their cousin, haven’t been invited to the marriage service but emotionally you feel very upset. Quite understandable. My cousin invited us to the afternoon meal at her wedding, ie the main and very expensive meal, but not to the marriage service which was held in the same hotel, an hour earlier in the day with about 60% of the guests attending it. I had thought that we were long term friends but clearly we were not important enough to go to the marriage ceremony itself. At the meal, an hour later, half our table had attended the service and said it was sublime, spiritual and amazing!
I was very hurt, although I had wanted to be grown up and I know that people can invite exactly who they want to all or part of a wedding, it actually upset me. If I could go back in time, I wouldn’t go to the wedding meal at all but would have made a diplomatic excuse and invited them to a lovely dinner with us and our immediate family on their return from their honeymoon.
Logic doesn’t always win out over emotion! Don’t feel bad about it, and please don’t let the replies from some people here upset you. ???