THE WEDDING is going to be at the home of the bride's mother, a home acquired during the divorce settlement, and the plans had been going well until the bride's mother got the news: Her former husband is bringing his brand new young wife to the wedding.

"I nearly died," says the bride's mother."I have this awful feeling I'll crumble and lock myself in my bedroom. My sister plans to says, 'Ooh, are you the homewrecker? We've never had one in the family before.'"

Weddings have always been tricky. There are endless protocol and etiquette questions and endless fights over who to invite.But the question now is not whether to invite your third cousin from New Mexico. Now, the question is whether to invite your father from across town and his new wife. If you don't invite them, he is sure to be offended, and she is sure to be furious, and worst of all, he might not pay for the wedding. If you do invite them, your own mother might decide on the spot not to come to your wedding. She might refuse to confront That Woman.

"I suppose I'll have to go and be a lady," says one mother who expects the stepmother to show up. "I'll have to act like a lady and be a lady because that's what I am and she's not. I'll have to go and hold the feelings inside me. . . . It really is a terrible thing."

She attended a wedding where only the father of the bride showed up. His new wife did not come, he said, because the bride was simply not her child. "He gave his daughter away and stood in the receiving line with his former wife, very compatibly. Most men, I think would not want their second wives there. She doesn't have any part in that family. If the situtation is such that she broke it up and took the father away from his children, it's even worse."

And it's worst of all is she is younger and Fench and beautiful and was the mistress before the divorce. The ex-wife in this situation expects to meet this stepmother at a child's wedding this month. "He left me for a specific purpose, i.e., another woman, and for no other reason," she says. "Something younger, taller, skinnier, gorgeous with a lovely Fench accent. And I'm going to feel like a short, fat, wrinkled frump at that wedding when she shows up and she's going to have the latest Dior on. I just plan to ignore her and pretend I've never seen her before."

The initial feeling here was that stepmothers, particularly those who are recent acquisitions, should have the good grace to stay home. This is after all, the wedding of a child she may not know well; the wedding of someone she did not help raise. But is that realistic? How is the stepmother going to feel, being left out of a major family celebration? How is the child who will visit the father's new home in the future going to greet a woman who was left out of that child's wedding?

It's terribly annoying," says a June bride whose fiance's parents were unamciably divorced and remarried, creating two sets of unamicable parents. "The adults involved in this mess are the ones who ought to be acting mature about it and they don't." The bride is trying to seat the bridegoom's various parents at a wedding rehearsal dinner, at the ceremony and at a formal dinner following the large wedding. "I've found it very frustrating and annoying," she says. "It's my day. It outrages me, but I'm trying to accommodate as many people as I can without hurting any feelings." Or causing any fights.

An advertising executive in Washington who is on his third marriage has brought second and third wives to the marriages of children by his first wife. At one wedding, all three of his wives were present."Everyone was very civil and it worked out all right. I danced with all of them and I danced with my ex-in-laws," he says.

"I think it's most difficult for the new wife who must come and stay in the background as much as possible. It's also difficult in terms of all the friends at the wedding because so many of the older friends knew both the husband and the first wife. And it's VERY difficult for the husband, who all during the ceremony and reception is looking over his shoulder to make sure he doesn't offend anybody, especially his current wife.

"He can't be too nice but he must be cordial to the ex-wife. But new wives have a tendency to overemphasize their husband's politeness towards the ex-wife. They see every amenity as a threat."

Should the new wife stay home? "That is entirely up to the present wife," he says, "but I don't think it is good for the current marriage if she decides she doesn't want to see her husband's children wed. I think it's a ludicrous idea unless she's completely divorced from her husband's children.

"Besides, most of the guests come to see that byplay anyway. It's like going to the circus and waiting for the trapeze artist to fall."

All this sounds terribly modern and terribly civilized. We know that good, honest feelings such as jealousy and vengeance and hatred have no place at weddings. Who, after all, in this age of multiple marriages and serial marriages has the energy to keep feuds going and to keep track of who is all right and who is the homewrecker? Weddings are occasions to put away genuine feelings about our friends and relatives and to be polite.

And yet, there seems to be something fundamentally wrong with a situation in which the mother of the bride does all the work and planning for the wedding, the father foots the bill and then shows up with a recently acquired wife who can't tell his children apart from one another. She may have gone from being That Woman to being his wife, but she is still an intruder. She is taking the spotlight away from the bride and using the wedding as a public occasion in which to assert her territorial claim.

Manners and etiquette, those invaluable frameworks of conduct that keep us from killing each other, now allow and even suggest that a new wife attend the ceremony. That's now considered correct. But there are some weddings and some circumstances in which the pain of divorce is still so acute, the wounds still so terribly open, that the stepmother might best decide on her own to stay home. It might not be modern and it might not be terribly civilized, but it surely would be kind.