I would say take OFF the white coat immediately and burn it!!! You are NOT their doctor.
They may or may not have mental health issues, but that's not an excuse for what they are doing. It's just NOT. It really annoys me when people try and be all liberal and understanding by assigning cruel and hideous behaviour to 'they can't help it they've got mental health problems'. It's really demeaning to those who actually have mental health problems and manage to behave kindly towards others. We really like to blame everything on illness for some reason... but you know what? Sometimes people are nasty, mean and selfish. Not poorly. And the other bit that I find unhelpful about the mental health narrative... These people don't want to get better, they are t seeking diagnosis, treatment, help. So yes, they may well be damaged, may well have things in their past that could be used to explain how they've become the warped and nasty people they are today but it's not an excuse or liscence to be a monster. And it's also not a forgone conclusion, how many other people have terrible things happen yet don't turn into cruel and manipulative power crazed aggressors?!
I'm not annoyed with you by the way! It's the unhelpful advice you're being given which leaves you as the in laws whipping boy, encourages your DP to not rise to his own protection, or yours, or his children. Without meaning to, people are assigned one rule to you, and another to the in laws. Forcing you be be the adult and not allowing you to react to the hurt and cruelty, and giving you no way out of the in laws control and dominance.
So I'd say a big fat NO to rising above it, trying to manage it, sacrificing yourself for some woolly notion of DP and DC needing a relationship with these poisonous horrible people.... All for what?!
Get the white coat off, and the wedding dress on.
I'd start showing your hurt, your vulnerability, your bewildered confusion and upset that people would behave like this.
Your feelings count. you count. And it's high time your DP stopped wallowing or getting caught up in his hurt. Yes it's awful but it's really awful for you too. And people need to STOP excusing the nastiness, and START being kind to you.
Or I may get very cross with them all on your behalf :)
The reason I'm so cross by he way is that my parents/ mother (not in laws) is highly toxic and has ruined decades of my life... Whilst other people helped keep me in the firing line and the status quo the same. They would probably be upset to realise that and try and shuffle away from any responsibility for the situation, but really, it's the people around them that allows these cruel and damaged people to cause so much damage and devastation. You wouldn't allow a child to rule the country right? So why do we let incapable and unpleasant people rule families lives?
Protect your DC absolutely, but also protect yourself. You are important and you matter. I really don't like the way that you are getting lost in this mess.