There is more to this story than you are telling us. A great guy from a great family doesn't suddenly tie himself to a miserable girlfriend. If this seems to him like an okay way to live, then his life before meeting this girl mustn't have been too great. He's had problems for a long time, which you, as his sister, know all about. She is not the first person to control him. Long before he met her, he learned to be passive.

So, in answer to your question, start by getting honest with yourself and trace out what really led him into this kind of a relationship. He is getting some deep need of his own met by being with her.