NC as I've posted quite a bit about this relationship over the years.
Two of my first cousins have organised a Christmas wedding and I refuse to attend it out of principle. There are millions, if not billions, of suitable partners for these two so why choose each other. We are all first cousins!
My parents, aunts and uncles and my siblings all intend to attend but I won't be moved. I know it probably won't make a little bit of difference to them (my cousins) but I can't go knowing full well how much I disapprove of their relationship.
My immediate family feel similar to me but will go regardless. They want me to attend and in truth there is nothing stopping me from going other than my dislike for their relationship. I know it's not unreasonable to not attend a wedding but I just had to get this out now that invites have come along.