I try venting to my mother in law about my feelings..

When you were venting, were you complaining about her son? My daughter-in-law used to complain to me about my son, and I really had nothing to say about her complaints. But she really was doing more than venting. It was like she wanted me to "fix" everything she thought was wrong with him. My attitude was, "He's an adult now, I did the best I could, and my job is now over. He is what he is and if you didn't want this version of him, you should have married a different person. Your disagreements with him are none of my business." Not that I said all that, but I refused to be drawn into the middle of things. I certainly didn't respond with complaints of my own, though.