Enid Is Sleeping (movie, 1989)

Enid only looks like she's sleeping. In fact, she's dead. Shortly after Enid caught her husband Harry in bed with her sister June, a row ensued, whereupon June accidently killed Enid. June's problem now is to hide the truth from the authorities, including her police-officer husband Floyd. Then she decides to reveal Enid's demise, albeit rearranging the damning evidence to make the whole thing look like the accident it really was.