Inviting kids and people that don’t get along to a surprise party?

I (25F) really want to throw a surprise birthday party for my fiancé (26M) this year. We have been together for 8 years and in the past 3 he has stuck by me through multiple health problems, so I really want to do something special for him this year.

The problem is I really don’t know how to invite everyone without making it awkward. We are in a townhouse so I am only inviting his friends and side of the family, but his dad and stepmom really dislike his mom and grandma (and vice versa). I’d like to think they are mature enough now to keep it civil, but I honestly have never seen them in the same room in all 8 years and I really don’t wanna risk any tension. On the other hand though, I know if I were to invite his mom and not his dad (or vice versa) either would find out and be upset. Fiancé also has 3 little brothers from his dad and stepmom (10, 7 and 4) who are all verrry rambunctious, and I’m worried that inviting them could ruin the surprise and make it feel like we are babysitting all night. But I’d feel bad asking them if they can find a sitter, especially if the rest of the family is coming.

TLDR: how do I go about inviting my fiancés separated parents to his surprise bday party when they hate each other but don’t want to be excluded? Should I invite his 3 young, rowdy siblings even if they are the only kids present, or ask his dad to find a sitter?