I'm so annoyed.

My boyfriends kids live with us part time (15 and 13) but they always argue about who has to clean what, even though my boyfriend tells them to do it before he leaves to work. I try not to get in it, because they just argue back and forth about what "DAD" said. I'm so annoyed at this point, and I also feel like if I say something these kids would just start to hate me. I do tell what to do most sometimes; but I know some of the stuff they do, they wouldn't do it if their dad was here. I'm also 21 weeks pregnant and just irritated. My boyfriend always gives me the right to say something but since most of the time I let them both just deal with their own problems, I don't know how to even say anything to do when they fight about who has to clean up.