I think that's completely valid. I'd like to hear other steps chime in on this, but even in the most ideal situations this whole "lying" thing can happen with the skids. My stepson lied the other day about the most stupid thing about three feet away from myself and my FDH.
He told FDH that *I* put his hand in the dog's mouth and "made her bite me". (the dog had jumped in between us because he was trying to ram into me because he thought it was funny. I ignored it, but dog is mine and got excited and jumped between us so SS whacked his hand on her face)

*ahem* genius, we're right here. both of us. nice try.

it is not unreasonable. i actually am in the that boat. SS10 is a disrrepectful snob who does nothing but backtalk, lie, and cry. i told DH if he doesnt step up they are both leaving as SS10 will not be welcome in my home. i already reserved the right that i will have nothing to do with SS10 unless DH is there. DH and BM made this mess of a child and if i have no back up i am not the one responsible for putting him in his place. their failure should not be your defeat, you know?