When someone lies about the cause of the murder of a family member it often causes the police to zero in on that person as a suspect.
With forensic investigative science being as good as it is these days (and even if it’s not as whiz-bang as the CSI series would have us believe, it’s pretty darn good when the state police Major Crime Units get involved as they usually do), it was probably very simple for them to determine that the story about “an intruder with stringy gray hair” was made up, and then start to find other evidence (such as the angle of the stab wounds, for example) to determine that the assailant was also not a tall person.
Putting together the clues about “we’ve been lied to by the victim’s brother” and “we know what we know about how the attack occurred” and (probably, later) “the brother’s story keeps changing”, clues them in like hawks chasing a mouse.