To put it in just a few words, If Jeremy was to inherit everything, which was likely, they would end up with nothing and not only their livelyhoods were in danger, but also the very places they lived. I'm not saying they did, but they had a real reason to put Jeremy in the frame for murder. The strange thing is that they tried from the very beginning to accuse Jeremy. That in itself is a very strange thing for a family to do? But then again since he was the "Cuckoo" in the nest? 
The crazy wallpaper incident comes to mind. More than unusual behaviour!

I said yesterday that Ann Eaton seems truthful in her witness statement, but during her cross examination at trial she gives three different reasons for, in a fit of rage, tearing down all wallpaper in their bathroom. You don´t have a fit of rage for three reasons - I know which one set her off: the land deal Jeremy talked about to her before the tragedy.