There were that family in America ..the Lawson Murders ?
Back in the 1920's ....if you can get hold of a book by Jonathan Goodman .. ..he wrote about real crimes and I'm sure he did a book called Christmas Murders.. Written a good few years ago now though ..you can probably get a copy from the library .
The problem is I've tried googling but haven't found anything suitable. Ideally I need a real murder that was the end of an interesting plot, hopefully based around a stately home or castle. I know it's a tough ask, but I heard this was the site that may be able to find it...