Oh Op, your 'D'P's family are toxic bullies.
And if I'm honest, your fiancé doesn't seem like the greatest catch. He lets you do all the wirework, probably all the child rearing and does not treat you with respect. He sounds very immature and not terribly bright.
I think if you marry him, you will always feel second best to his family and that's not okay.
I know you say you got on well during lockdown without his family but he clearly hasn't changed. Where his birth family are involved, you will always have to deal with the fallout. And clearly his family will always be around.
Are you very very sure you want to marry this man? This might be a good thing in disguise. Imagine living in a home which won't get trashed by drunken idiots? Imagine knowing that you're safe from guilt tripping and manipulative behaviour.
I think you posted tonight because you know in your heart you ANBU and the inevitable conclusion is that your DP and his family ABU and Will Always Be Unreasonable and you'll be expected to put up and shut up.
I'm worried that marrying this manchild will be a dreadful mistake. Yes, I know I don't him but no amount of good points can ever make up for the shitty, selfish behaviour.