21yearoldman, do you really want to know why people (I'm thinking that this post has more to do with your OH than anyone on here, btw) can't just shut the door on abusive parents, just like that? Because if you really want to know I suggest you try reading up on pyschology, or ask people who've been there from a perspective of "I am ignorant of this and would really like you to educate me". And accept that it is a very complex subject which can take a lifetime's experience to properly understand.
As it is, you have asked from a perspective of "there is a simple and easy solution to this, and I have it - even though I have no direct personal experience of what any of you are talking about! If you would just listen to me, and do as I say, all your problems would be solved!" I'm afraid that comes across to me as disrespectful and unhelpful, tbh, as well as emotionally very naive.
There are very many, very real and very, very deep psychological reasons why it is so very, very hard for people to shut the door on abusive family members - far too many to encapsulate in one post. And for the record, many people (usually ones that have been groomed to accept and expect abuse from childhood) do take a lot of emotional abuse from partners and friends in their adult lives as well, there are any amount of threads on here that are testament to that.
I am someone who has shut the door and walked away, and that is precisely why I know how difficult and painful it is to do, and why I would never, ever judge anyone who doesn't feel either able or willing to do that. And contrary to what you think, while it can minimise future damage, shutting the door doesn't actually stop the hurt, anyway.
Living with the knowledge that your parents don't actually love you is incredibly painful no matter what your relationship or non relationship with them is. We are human beings, programmed from birth to love our parents, and to want and need their love. Especially our mothers. It is a primal, visceral thing. Think about it. One's mother is supposed to be the source of all safety, all love. Do you have any idea how it feels to formally relinquish the fantasy of one day actually having your mother's love, especially when the myth that all mothers naturally love their children more than life itself, that maternal love is an inevitable consequence of physcial motherhood, is plastered all over the world we live in and reinforced culturally in every conceivable way?
Your OH hasn't reached that point yet, and maybe never will. Maybe it's not the way she's going to deal with it. Who knows? I can understand how it's hard for you to see her being hurt over and over again and apparently just not getting it, not getting that it's never going to change. But if you really love her and really want to support her, you would do better to show her some real respect and understanding and allow her to come to her own decsion in her own time. It's not for you to force the agenda of when or if she should cut off from her mother.
You can talk to her about your feelings - tell her how frustrated and impotent and unhappy it makes you to see letting herself in for this repeated hurt, if that's the truth for you. Tell her you're frightened her mother's power over her will one day force you apart, if that's the truth for you. Tell her you're scared for her, you love her and she doesn't deserve to be treated this way, tell her she's worth more than that - she probably needs to hear that. If you think you would one day have DC together then obviously there are a lot of issues to be discussed around that, too, from how this would affect her parenting, to what relationship any DCs would have with your MIL.
But please don't assume that you can just wave the magic wand of your apparent "insights" (I'm sorry to tell you, but they're not really insights - but if you are really a 21 year old man, you can easily be forgiven that) and make it all go away, just like that. Life unfortunately isn't that simple. Tbh, it's a big fucking mess a lot of the time. What you're actually doing is undermining her and telling her she's doing it all wrong - hey, and that's just what her mother's been doing for all these years! Do you really think that's going to help?
One last thing - it's interesting that you've chosen someone whose family dynamics are so screwed up. Perhaps there's something your unconscious is trying to tell you?