I'm sorry your DIL has hurt your feelings so much. I don't think I can help with that one, but I wanted to share a perspective that may be able to help you regarding the way she behaves toward your DS.
I was thinking about myself and my DH, as well as other couples I know. DH and I love each other very much, and there is no doubt in my mind that we make each other happy and will have a long successful marriage. But, like everyone, we are not perfect individuals. We both have bad habits. When we are comfortable and alone, we both do and say things that others might find obnoxious. Sometimes, for example, I whine for him to bring me things, and he jumps to it. He has bad habits, too, but I can't even think of any right now because I do not let them bother me, but instead love them. We have learned to love everything about each other, including the ugly stuff. If there is something really big or hurtful, we know we can bring it up, and then we have to decide whether one needs to change or the other needs to just accept it. This is why I would never sign up for a reality TV show. Viewers would be able to pick our marriage apart in one episode, taking sides and counting flaws. It's kind of the same thing when someone stays in your home for an extended period of time. You try to be on your best behavior, but eventually you get comfortable and revert to your true self, and others are left to judge.
My point is, maybe in a wierd way, your DS likes your DILs princess (even controlling) behavior. I know there was a time when you knew DS as well as I know my one year old DS, but he is basically a different person now, and his preferences might surprise you. You just have to trust that if DIL is really hurting him, he will be adult enough to handle it on his own. If he isn't changing it, then it must not be bothering him, and you shouldn't let it bother you.