My fiance's parents hate eachother.. what to do!!!?

background: My fiance's dad left when he was 14 and his brother was 12. His dad cheated on his mom with his secretary and they were both married at the time. he came home one night and said he was leaving. at that point, his mom had no job and was a stay at home and was living off his dads 200,000 a year. when he left she was screwed besides what he paid monthly, but at the same time i dont feel so bad for her because she got a job as a secretary and that was where her search ended even tho she has a degree.. anyway, his dad and mistress moved in together and my fiance and his brother absolutely hated the mistress because their mother like forced them to. My fiance went to rehab in the first five months we were together and when he went his mom was not letting me go see him and not updating me... i called his dad and let him in on the info even tho i had never met him. ever since, my fiance has been grateful for his dad and stepmom and his stepmom just had a baby. oh they also got married without anyone knowing.. anyway, we spend a lot of time with them now and his mom is in a new relationship... well has been for like a year. his brother still hates his dad but it also has a lot to do with his mom.. his mom refuses to be in a room with his dad even tho his dad really doesnt care... they have only seen eachother in court i think... what do i do? my fiance wants them all there... if i had to choose, im partial to his dad and stepmom and baby brother only because they are ALWAYS there for him and his mom kicked him out and signed him out of HS... she has peculiar behavior that i dont care for and she treats me bad even tho things have gotten better... how do i keep them both in one room... i will be inviting her new beau and his wife and child... also, his brother will be his best man so his brother mom and her beau are not optional. thanks!!!