Are they aware of her deterioration, and the full extent of her condition? Some families, sadly, aren't that much into keeping in touch, even with their parents. If they are already fully aware of what the situation is, and still can't be bothered to make the effort to see their mother then, unfortunately, there's not much you can do, and little point in contacting them. But, if they aren't aware of how much she is deteriorating, they may thank you for letting them know, before it's too late.
Personally, I would drop them an email and simply say that you thought you should update them on their mother's condition, as you can see she is deteriorating. Tell them you understand if they are unable to visit, but if they can, then now would be a good time whilst there's still chance that she will remember them, and you're sure it would make her happy. Just tell them that you thought they should know. Whatever they then decide is up to them.