Trying to Locate a Scary Book, Recognize this Symbol?

This has been driving me nuts. See the above symbol? It comes from a book I read back in elementary school, juvenile fiction. I can't remember the name of the book! Hopefully somebody can tell me the name or author of the book if I describe the story. Main characters are an older brother, younger sister. I think they are pre-teenagers, but just barely. Brother gets a job mowing the small town cemetery's grass. Sister tags along, because the graveyard is cool and creepy. The brother makes wisecracks about the various people buried there and makes up a series of satirical rhymes using names on tombstones. Then we are introduced to a mystery -- there is a mausoleum, or large gravestone with an angel on the top of it. The angle points towards a part, or corner of the graveyard, where a certain plot is. This plot is where a family is buried, reputed to be witches. The person who put up the angle blamed this family for the untimely death of their son. The kids examine the cursed plot tombstones, but there is not much of interest. Then, before Halloween, the kids notice that someone has drawn a symbol on one of the gravestones. In red paint, I think. This (above) is the symbol. Then some stuff happens, the kids have their eye on the last living member of the "witchy" family -- an old woman who they are naturally very afraid of. The girl ends up getting kidnapped by the old woman, who turns out to be a witch. The old lady tries to bargain the girl's soul away to a demon the witch invokes, but instead the demon tricks her and turns the old witch into a Douglass Fir. Ring any bells? Book had some illustrations in black ink.