OP - really think twice about the future of your relationship. I say this as I’m in a similar situation as you, but 20 years down the line!!
DH’s parents divorced over 35 years ago. FIL has been married to OW since then. MIL and FIL have only been in the same room ONCE during that time (they only ever communicated through solicitors before then). You’ve guessed it, the only time they have encountered each other was during our wedding! DH was so concerned about having both parents in the same place that it took 10 years for us to get married!! We got engaged fairly early on, but he then refused to even discuss planning a wedding. It was awful. FIL’s wife (OW) caused a massive fuss before the wedding, at one point refusing to come, so for months before, FIL said he would come to the church, but not the reception. They’re such fuckwits. Eventually they all came to the wedding and ignored each other 🙄
Now we’ve got our own DC, it’s a ridiculous situation. Every Christmas, birthday or event is fraught with emotion and family politics. I feel sad that my DC have never known proper grandparent relationships and we have secret bbqs etc where one side isn’t told. At Christmas or birthdays, DCs receive lovely presents from FIL/OW if we spend the occasion with them, but really useless and paltry presents if we spend that time with MIL. Yes, they really are that bitter and spiteful! I don’t think DC have noticed. DH won’t go NC with FIL, even though I’ve tried (now me and DC are LC with them).
With hindsight, I should probably have read the signs many moons ago and married into a more functional family.
Sorry for the essay. I just wanted to share the experience, as I don’t expect your DP’s issues will just be contained to a housewarming party. It will likely effect your whole future and that of any DC 💐