Okay, lots of replies which is great, the Martha has me baffled as well because I can’t find a sister named Martha unless she is a cousin. William’s headstone would have been placed sometime after his death , obviously, and whoever did may have got Eliza’s death incorrect which would explain 1844, but it seems she was still alive in 1851 census. The only conclusion is the headstone is wrong. More searching for Martha is required.
Regards
Craig