my stepdaughter and fiancé ☹️☹️☹️

My step daughter is a mal-mannered brat...I know lots of 3 yo girls can be, but my fiancé just lets her act spoiled. He wants me to play 100% mom role, but gets irritated with everything I ask her to do. Like, she can't have candy before dinner, she needs to eat ALL of her dinner before she has desert or candy, a meal is not a slim Jim... 
and today the cherry on top is if she's eating in (our) bed (which I already HATE), she needs to use a tray bc she drops and spills things CONSTANTLY. I have become more strict since my daughter was born in April. I want her to have manners and some traditional home training... I don't want her to leave these bad habits from her big sister... mind you, there are other bad habits that I have not mentioned in this post. I'm irritated that he wants me to step up so much since her mom is a party girl and never takes responsibility for her, so when I try to and lay down rules and try to have some stability and home training  in our household, he gets so defensive of her and I'm sick of it. He isn't helping me parent.. he's not on my side as a parent. I would rather leave than to put up with not being taken seriously by him and my step daughter.