Wtf?! Seriously?!?

My fiancé has kids from a previous marriage. The girl is his, the boy is not but he raised him since 6 months old and the bio father is in prison.   The child is the most difficult, disrespectful, manipulative child I have ever met.  If he doesn't get his way prepare for 1.) tantrum 2.) wants to go home to moms house. 3.) tells mom things about me that aren't true. 
I swear it's like this kid is trying to split us up.  He was fine with it in the beginning when my fiancé first talked to them about him dating me & living with me.    Now suddenly it's one thing after another. 1.) telling his mother my fiancé & I argue all the time and it's scares them. (We never argue in front of them, never have) 2.) telling her he wants them to get back together. 3.) and now today telling her I'm being mean to him when he was asked 100 times to clean his room by me and his father. 
This is my house and I'm not going to keep being disrespected.  Im getting to the end of my rope.  Rant over.