Not sure what to do.

I have been married now for 10 years to what I thought was a wonderful and caring woman. One year ago my step-grandson moved into our home because he was having problems dealing with being picked on by his siblings all the time. Anyway, now that he is living in our home he does not have to pay rent, does not have to do any house chores, and basically does little to nothing except play video games in his room all day if he is not sleeping. My wife seems to think that asking him to help out around the house is just being disrespectful and mean on my part. She says that he is a good boy and that he deserves respect and that to ask him to do anything is going to hurt his feelings. My step-grandson is 20 years old, and he has lived a sheltered life with his mother, so he is not very good socially, and is very immature for his age. The problem here is that my wife has been accusing me of being very insensitive and mean to him because I will at times tell my wife that I wish that he would help out around the house more. I have also talked to my step-grandson about this one on one and in a way as not to upset him. Soon after we talk is when my wife usually starts accusing me of being disrespectful and mean to the boy. At this point my wife and I are still together, and I keep trying to work things out with her to at least meet me half way on getting my step-grandson to help out in exchange for room and board. My wife still insists that he does not need to earn his keep, and will then be very defensive of him saying things that are really not true at all about me. I feel that if this issue does not improve at some point that our marriage is going to suffer greatly which I do not want to happen. I am at a loss as to how to handle this situation except to possibly leave or separate from my wife for a while. It does seem obvious to me that she values my step-grandson's company more that she values her own husband's wants, needs, and feelings. I feel like I'm ignored most of the time anyway when the two of them are together. Someone please help! I need advice quick.