After reading your later comments on this, I think I'm with wednesday. She needs to get psychiatric help. If she is being physiaclly abusive, you can probably force her to get help by charging her with assault and battery. In most cases of this sort, the judge will give the defendant the option of hospitalization over jail, particularly if you, as the victim, request that.
This is going to be very hard for you to do, and, in a worst case, it might even bring about the end of your relationship with her. But the alternative is to continue to accept this abuse given to yourself and others for as long as she lives. I'd suggest talking to a lawyer, if you can afford it, and a free legal clinic if you can't. Describe the situation as you have here, and ask them what you can do to force her into a situation where she can get the help she needs.
The worst thing you can do is cover for her with outsiders. If I had to do my first marriage over again, I would not have shielded my (then) wife from the consequences of her actions as I often did. It's like what they say about an alcoholic. If he passes out on the kitchen floor, and you pick him up, clean him up and tuck him into bed, you are only enabling his further drinking, and making the situation worse for both you and him. The best thing you can do is leave him there and let him deal with the natural consequences of his actions.
Frankly, the fact that people like these can control themselves when others are around, and act out only behind closed doors (my ex was the same way), leads me to wonder if there is not a degree of deliberateness in what they do. They may be delusional, but they are not so far detached that they cannot see how convenient it can be to be crazy. You can get away with the most outrageous behavior, and people just chalk it up to the fact that you're looney. If you buy into that by covering for her - by keeping it the family's "dirty little secret" - you are enabling her behavior. Don't do it. This sort of behavior needs to be exposed to people who can provide help, and she needs to be forced to get help - if necessary, by being committed against her will to a facility where help can be provided.
As I said, it won't be easy. Good luck.