I cannot answer for his sister, but I certainly can answer for myself. It happens slowly, it's insidious, how one gets involved with someone who is like this. First step, is recognizing it for what it is, however. Silent treatments, begging for forgiveness, soothing anothers insecurities, never seeming to have enough love to give them for them to believe it. Of course, I've born kids with a person like this, and still must deal with them. Recognizing what brings you to involvement helps tremendously. For instance, there's a people pleasing trait that exists. Rescuing tendencies, as well. Nurturing/Protector roles. There isn't one specific, first step to be taken. There's a multitude of steps, that need to be taken. It could take years. Maybe not years, to release their clutches, but years, to sort yourself out, and not repeat this role again, in the future.
It took, a lot more than just learning to stop walking on eggshells, for me. Seeing it, for what it is, without playing the role of victim, is probably the most important step, one can take. The rest, all the books, therapies, support groups, etc, just strengthen a person.