Your DH did what he felt was best for his son. Just like you state you did what was best for your daughter. Both children are his, but he had the advantage of knowing his small daughter would be perfectly safe and happy being left in your care. 

His son needed him. He was there for his son and with the therapy and love provided by Dad the young man is turning around and healing. If the shoe was on the other foot, I seriously doubt you have have left your daughter down the road in a facility when she was at her neediest.

You probably would have been better off , marriage wise, if you had suggest DH rent a small apartment and tend to his son , giving his son that much needed sense of finally feeling safe, cared for and loved while working on the traumatic experiences the boy had been through. While you kept the home fires burning. It was the idea of sending the child away that struck a damaging cord with your DH. Even if you truly meant well. 

Have you suggested Dh and you now do counseling (both marriage and family counseling) to try and heal as a family and bring the family back together perhaps within time? 

You weren't 'wrong' for wanting to be certain your daughter was safe and wasn't exposed to such heavy issues the brother was dealing with. But in trying to do what you thought was best for everyone, you somehow made the Dh and other children feel rejected. Just as the young son needed time and healing, so does the mending of the marriage and the repairing of the damage to the family. 

It was a very sad and stressful time for the abused child and very emotional for your DH. I imagine he felt like he was being asked to pick between his kids. He knew the little one was fine with you, so he did what he felt was best for his older child, who wasn't fine and in need of him. Not moreso than your daughter, but rather very differently.

I don't know if your marriage is gone, but I do know your one child is no more deserving or entitled to have you and Dh together in one home than your Dh's other children are deserving of having their one parent there for them. Don't use that argument on your Dh about how deserving your daughter is over all his children. It won't set well with your Dh. 

At the time, neither one of you were wrong. You both did what you both knew had to be done. But coming back from it, won't be easy. 

Welcome to steptalk. \

*** I edited just to clean up my typos 

Counseling is a great idea.

While you're there, ask how the hell it "slipped" that you suggested boarding school for SS. That seems a little hard to believe.

If everyone knew every awful thought or utterance we have about each other, there would be few intact marriages or families.

I don't believe it was a "slip." But even if it was a "slip," whoever made the slip bears part of the burden for your broken family. Children do not need to know what is said in adult discussions about family management. Someone injured you when they "slipped."

That said, there's no reason this needs to be the end of your marriage if your husband starts looking at your suggestion for boarding school as a practical matter rather than the end of the world.

You made a suggestion out of concern for all the children. Your husband rejected that suggestion and took another route. Thankfully, his route turned out well for SS. But if the whole family is going to hold something against you that none of them but their father should have known in the first place, I'm questioning who sabotaged the marriage by pitting all your stepchildren against you.

Your husband is an adult. Perhaps you had a chance of convincing him your suggestion came from concern for the children and it should be put behind you. But no one can expect children to have this "big picture" view when they are just kids.

If you think any of the kids has the maturity to understand your explanation, maybe try talking to each one alone. Ask them to forgive you for suggesting something that might have upset them, but say you hope they understand you were just trying to do what you thought best for SS. I just don't know if kids that age can understand something that complex. Perhaps you could start by chatting with your in-laws, who might understand why you suggested it, and then they could help the kids to understand it.

I think counseling is a good idea to help the adults get past this and figure out ways to mend things with the kids.

But if your husband was the one who slipped, don't let him off the hook for that. Something like that doesn't just slip. He made a conscious decision to tell children something meant only for adult ears that would pit them against you. He needs to bear some of that too.

Your husband chose his son over his marriage, over you and your children together. If he loved you truly and prioritised your marriage, he shouldnt have left in the first place. Was leaving the ONLY option he had?  Did he sit down with you to discuss SS living with you guys after you suggested boarding school or he just up and left without trying to come into agreement first? You were not wrong to give a suggestion of what you thought best. DH was wrong for telling the skids that you wanted ss out of the house, obviously the kids would let it slip to the whole family and they would all judge you and see you as an evil person without even hearing your side of the story. No one is perfect, we are all to say and do the wrong things that we will later regret, that is why we are to practice forgiveness. A spouse has to cover his wife's mistakes, resolve your issues between the two of you and maybe a therapist and not let anyone hear of them especially the kids and in laws so that his wife stays respected and loved and not judged and hated. Fact that the in laws are not your relatives but his, they didnt grow up with you or know you very well so it will be very hard for them to forgive you and have unconditional love. 

I'm sorry to say that your DH does not value or prioritise your marriage like he should, he has already chosen his kids over you, they matter more than your marriage, if you ever live together again problems will arise as he will always be ready to leave you and stand with his kids and whenever the skids give you a problem, you will be afraid to speak out and now that the skids know their dad chooses them over you, they will take advantage and be a nuisance knowing there is nothing you can do, their dad will always defend them and choose them over you and the in laws will accuse you of being abusive. I guess DH didn't mean it when he said for better for worse. He doesnt understand that as his wife, you are his better half, you are his first and utmost priority. He wasnt supposed to leave but compromise for the sake of your marriage and agree on something that would work for the both of you. He cannot say if his son is not welcome then he isn't too, you did not marry his son.

I think you should go to therapy and your husband should fix his mess, he should talk to his skids and family that he was wrong to say you don't want SS, he misunderstood you, he should tell them you are his wife and he loves you, you are important to him and you care for them and you will be moving back together and they should respect you and be polite. DH is the only one who can fix this because he is the one who made the mistake of leaving. 

DH did what he thought was right, and so did you.  Although the result was and is difficult, it may have been for the best- rather than have a young man who was acting dangerously in your home.  Counseling is definitely a good idea.  Perhaps you and your DH first, then the entire family later.  These things can be worked out.  If SS13 is in a much better place now, family counseling should help you all out. 

Family counseling so that OP has an opportunity to explain what was clearly a miscommunication between her DH and herself.

Boarding school wasn't meant to be a rejection of SS but an opportunity for him to get intensive therapy that she thought he needed after the abuse he suffered.  And.. while it might sound silly now when SS is doing so well, she was afraid that he might do harm to someone in the family because he was doing very threatening things.  In hindsight, the worry may not have been necessary, but at the time, OP had no way of knowing that.