My therapist says if I want a relationship with my son, I pretty much have to take it and just basically let her be a queen.
While I am sure your therapist means well this is very bad advice. It sounds to me like your therapist has some unaddressed codependency issues — which is sadly very common amongst therapists. The reality is that you will be enabling your DIL’s illness by following your T’s suggestion.
It sounds to me like your instincts to draw hard boundaries are spot on — you are not being ‘mean’ to do so. You are not asking for too much to want be able to have peaceful family get-togethers. What you have said about having an honest conversation with your son where you tell him how much his wife’s abusive behavior is hurting you sounds like a mature way to go about communicating this to him. Yes, it is unfortunate for him, but your son is going to have to face the consequences of his decisions, and if you continue to act as a shock absorber for his wife’s anger, you will take a lot of the heat off of him which is not is actually not in his long-term best interests.