💯 this.
Don't marry him.
Don't have DC with him.
He's not a keeper.
If you stay with him, all your Christmas Days will be marred by arguing even if you don't go, because he wants to be with his arsehole family and you don't.
Your future DC will be exposed to this and led to believe it's normal, not a message you want them to receive.
You'll also be in a position of either going so you can take care of them or DP taking them anyway and none of the drunk fuckers being able to care for a child and keep it safe, never mind the fact you really don't want DC around such abusive people anyway.
It's absolutely bizarre that DP even wants to see them at all, never mind in that state - and to describe it as "enjoyable" is next level bat-shittery. Take it as a sign of how psychologically damaged by his upbringing he is and run for the hills.
You're best off staying single forever than getting involved with someone like this. You'll be opening yourself up to abuse and neglect by him in the future. Why the heck would you accept Christmas Day alone because he chooses his shitty family over nice and lovely YOU, the person he's supposed to love and cherish and who is supposed to be his number one person?
If you're not his number one priority on Christmas Day, you also won't be his number one at any other time. Like the birth of your future children or when the baby and toddler years get hard. He'll be inviting people round when you've not yet recovered from the birth because their desire to celebrate will come before your need for rest. He'll be down the pub or the gym whilst you're child-wrangling, doing all the laundry and trying to fill in that form from the school.
He's demonstrating that he'll never be wholly in the relationship, never cut the apron strings, he'll always have one foot in the camp of his family of origin. You can't form a solid relationship that way, you'll never be you two united against all others, solid in the face of adversity, because he'll allow his family to pull him in another direction away from what's best for you and his DC. He'll go along with it because he'll want to appease them for an easy life. Look how he behaves when his sister is nasty. No standing up to her, no calling her out on her behaviour, no removing himself and his girlfriend from the shit-show. Nope, he just tolerates it.
Also, if you ever get divorced, this is the type of reaction you'll face from him - because getting drunk, screaming, shouting, swearing, picking fights and being unreasonable is apparently all just fine and dandy.