Seven for a Secret

Gerry Llewellyn's mother is teaching him death magic, and his grandmother thinks he might help the family take over the world. He's eight. Waning magic in their land, and a legendary curse, meant British demon-bred witches were unable to enter the Otherworld, source of their strongest magic, for 200 years. Gerry and his father are the first seventh-born witches in all that time, a signal the curse may soon end, heralding a new era of greater power for them.
But even his family can't agree whether this is a good thing. They have peaceful lives, blending in with the rest of humanity, doing much as they please in the slightly more tolerant postwar years of the 1950s. Are vague promises of fabulous divine gifts worth upending that?Already scarred by family secrets and politics, precocious Gerry hates being treated as a miracle child. And now Gerry's eldest brother is getting married, so the Llewellyn children have lots to do besides dealing with their emotionally damaged parents, death demon siblings, and a sundry lot of fractious relatives.
They are all about to learn that seventh sons are special all right, but it's their mothers you really need to watch out for...