If your family isn't named it will be unlikely readers will link the contents to your family. Furthermore, about this book.... do you really think people are going to read it? No proper publisher worth their salt would touch such a volume. As smaller publisher for fly by night operation is unlikely to garner much of a sale. Frankly, I don't see people reading this in the first place. At least, it won't be the people who count.

She is 45 and a big girl. If she is unreachable in accepting support, you might want to at least temporarily cut ties. If she is of danger then contact the authorities. If you truly feel this slander is going to affect your family negatively get a lawyer.