I would strongly advise not to go to the wedding service. Inasmuch as some may consider this harsh advice, your attending this service would give scandal and the impression that you are condoning the fact that your brother has separated himself from Christ and His Church.
Jesus never said that it would be easy to follow Him. In fact, He sadly lamented the fact that following Him would bring division among families, brother against brother, father against son, etc. While family is important, we have to realize that God comes first. What your brother is doing is wrong. Even though you still love him, his actions should not be condoned. Pray for his reversion, but, do not assist at the wedding service.
While I don’t agree with your assessment that attending the non-Catholic wedding of a family member who has left the Church is going to automatically give scandal to anyone except those guilty of very rash judgement, I agree that someone who sees the situation as you do may very well be conscience-bound not to attend.
When there is a question of giving scandal, in other words, it is usually a matter that has to be discerned on the ground, taking into account what the attitudes of those in question actually are. I am speaking of when we believe it permissible to attend a wedding, but are concerned that someone else might take serious scandal, drawing improper conclusions about our attendance.
Where I am, nobody takes attendance at a wedding as endorsement of the religious choices of those involved, but rather as an act of tolerance, made for the sake of the couple. It would only constitute an endorsement of the marriage itself, and even then, not if it is a family member that is getting married. In that case, since attendance is
de rigueur, nobody holds it against you. The number of people who have had to sit through a wedding binding them to a new in-law that they never wanted is legion. Nobody is going to think you are all of a sudden OK with divorce because you attend the wedding of your divorced-and-remarrying brother. They know you go because, like it or not, this new person is going to be there for every family gathering until both of your parents are buried, and you have to get along with them. To refuse to attend would be seen not as an act of conscience, but as a kind of a public self-indulging of anger that your brother’s life isn’t under your control. It is interpreted as making a scene, and not a virtuous scene, either.
As I said, that is the way it would be taken here in Oregon. Maybe you don’t go because your conscience doesn’t let you, but you don’t stay home from a wedding because someone else’s opinion is going to smite you over it. It isn’t seen that way. Attendance is about tolerance, then, and not endorsement. Your actions are about what you have decided you cannot or cannot tolerate. Other people’s tolerances or intolerances are their choice, and their problem. If you don’t attend your brother’s wedding, it says you are having nothing to do with letting this new person into the family. It is really and very publicly throwing down the gauntlet, and it would be hard to make it otherwise, without the full cooperation of the family member and their intended. (Meaning: the couple is OK with your absence and does not care what anyone else thinks.)
In other circumstances, though, if regrets are necessary, it is possible to tender these regrets with gentleness and compassion, too. While a Christian may have to absent himself even if the decision causes division in the family, it is not necessarily true that it will do so. It is possible to agree to disagree without mutual rejection as a fall-out. Not guaranteed, but worth a try.
I don’t think that anyone here was suggesting that this is not a big deal.
sometimes hard to know, it helps to have discussions like this, though. I appreciate your thoughts.