If someone in your family gets married, to my way of thinking, that person become family...not 'family,' or so-and-so's wife or husband, but actual family.
What a lot you could be missing out on when you don't embrace someone as family who really is. And if you spread the feeling to others that the person in question isn't really family, it will proliferate, and to keep the peace they might embrace or temporarily adapt in big situations (oh we can't invite such and such to the party because A doesn't like him/her (even if everyone else does).
What results is:
A). The breach you create is huge (I know there are a rare, select few who relish in this breach because they are so selfish it feeds a basic need).
B). The person being ousted by the 'real family member' will permanently keep an arm's distance from those whom he/she would have really built up trusting, loving relationships with (so you have robbed your loved ones).
C). It robs people of what the sense of true family is about (why would ANYONE want to do that?). I have seen my husband live with heartache from one particular incident that he never should not have had to. He's never told a soul how much it hurt him, but me. I asked him why. He said, 'It doesn't matter, wouldn't change anything anyway.' And he's right, but it doesn't make me hurt for him any less.
I know people adapt, make their other relationships stronger, and in order to stay sane keep a very healthy distance from those who enjoy turmoil, but every once in awhile, that breach will sting no matter how strong you are. Today is that day for me.
A short time from now I will be fine, my head will be clear, and I will remember how great it is to not be a part of things that bring hurt to us. But for just this second (in spite of it being something that would make one person very happy), I'm going to let it sting because I have to miss an event I would have liked to have been a part of...in another life.