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Dear Concerned Parent,They may not still be together come Father's Day, as that will be six months off. I don't agree that adult children should always be put first, but this conflict isn't really about that. It's about conveying to your daughter that you accept her as a person in her own right. That you believe in her, even when she's not sure she believes in herself. She's the only one who can decide if her marriage is working for her, not you.
You love your daughter and you want the best for her. That means you need to follow her lead in this. As parents, we're always working our way out of a job. We encourage them and try to teach our children what they need to be strong adults, so let her do that here. She's in an unhappy situation with her husband and, as difficult as that is to watch, you have to take the back seat on this one.
Unless her husband has physically attacked her or you--or stolen from you--you have to let her decide what family functions to include him in. Whatever she does with the marriage is her decision. You may be disappointed in your son-in-law. You might have felt close to him, but are now repulsed by who he's shown himself to be. It's still her call whether she continues to be married to him or not.
In the past, my husband and I talked of traveling over the Christmas holidays so we wouldn't have to deal with our troubled children and their troubled relationships. We didn't end up doing this, but I very much understand your desire to avoid interacting with your son-in-law. You need to suck it up, however. Smile at him and sincerely wish him to find whatever he needs. Even if you don't like him--or he doesn't seem to like you--hope for his learning.
You didn't want your parents deciding who you should or shouldn't be with. Now it's time to let your daughter make her big decisions.
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DON'T TAKE THEIR CONSEQUENCES
It's very difficult to watch your child struggle, even when she deserves it. Parents frequently wrestle with the question of what they did wrong when a child of theirs makes a choice that leads to lousy consequences. As a parent, you had your difficult moments, but you shouldn't accept responsibility for the actions your child takes. The older the child, the more she gets to direct her life. This is her right and her responsibility.
When a kid is under legal age, parents have the role to direct her, but when you have an adult(or nearly-adult) child, you need to step back and make sure you're not stealing your child's learning.
When you make her consequences go away, you're robbing her of a learning opportunity. Don't do that, even if you have the kindest motivation. Even if watching her struggle is painful for you.
It's just part of the cost of parenting.
When I watched my first child learn to crawl and saw her heading away from me, I had a clear moment of seeing the future. No matter how much they love you, they need to lead their own lives. You will always be important to her--kids love even parents who abuse and neglect them--but you have to find a way to develop a new role. Be a loving, supportive parent who believes in her, even when she doesn't believe in herself.