I am so sorry that so many are going through this experience and trying to cope with it-it must be overwhelming. People run away from difficult and horrid things. They don't want to deal with anything uncomfortable or unpleasant. The fact that so many families are willing to let their child be abused or question their word rather than confront the situation is heartbreaking.
You can accept your family coming over for your son's sake, but it's okay to keep your distance in the future while you sort out all of your feelings about their nonreaction. It took guts for you to confront them and they should have to deal with the situation, even if it is years later.