How do I address "My Daughters vs.Her Daughter" issues?

Remarried for 3 yrs & have 2 dghtrs (13&11). My wife has a dghter (20), who lives at home for free. She doesn't help out much, leaves messes all over, refuses to do certain things, is disrespectful, shows a lack of appreciation for what we do for her & whines like a child when it is convenient. She has been coddled her whole life & takes admittingly takes advantage of that. My wife is oblivious to this. She thinks that she is being the "Good Mom". When I try to address this, she throws my children into the equation & says that they do the same things. (That is not entirely true) Then the vicious cycle begins. To me, the age difference & the fact she lives with us says it all. My kids live 3 hours away and when they visit, I expect alot from them & for the most part, they do well. I cannot stay on them for the entire visit. When this happens, my wife & her daughter will team up against me & my girls & make hateful comments. Rarely any apologies. My kids are afraid now. What can i do?