Think again, think harder

Rating: 9/10 (11 February 2020)

Marta the nurse is the killer, she played everyone included Harlan and the detective, if you still believe she was innocent and know nothing about the Harlan's will, which gives her no motivation, you are the one of dumbasses as well.
First thing first, what happened in the Harlan's bedroom at the night he was dead is from Marta, she told the detective and showed to us. Great, if you really believe that BS, how is it a lie, you may ask, she can't lie for she will puke, just as she deliberately showed to Detective Blanc and the policemen during her first interview. Brilliant, I must say, she played everyone like fiddles. So, what really happened?
She perhaps, not merely "friend" of Harlan, dare I say, she is a lover of Harlan, what they did in that room including "what you know" between two lovers would usually do. Therefore, the only logical explanation why Harlan changes his final will when he is fully sober is that he loves Marta and think Marta loves him as well. For Marta she probably knows the will from Harland, at least she outsmarts him so she can tell from the daily conversation and Go playing.
At that night Harlan was dead, Marta can tell she inject the right medicine, once again, in the final of the movie she can tell the slight differences between them, so she can definitely tell the nuances at that night. She knew someone switched them to play dirty, she decided let them play, she maybe causes the Harlan's suicide or she cut his throat by her own hands, we can't know because Harlan is dead. She sneaked back to home and waited Ransom to play, by the time she might not know for sure it was him, anyone in this family was possible.
What happened next was almost exactly she expected. We have to notice that since beginning of the movie, she dismissed her mother's complain about her sister listening the murder show, if she was worried as heck about her misdoing caused Harlan's death, she should at least show some emotions and worrisome, however she showed nothing but calm, she is a cold blood murder or player.
In the end, the dumbass Blanc and the others were convinced she was decent, smart, innocent and "lie to puke" nurse who deserves everything, and Ransom attempts to murder Harland and murders Fran, so he should go to jail, and rest of family are bad, which I don't deny, so should move away and live in poverty. Just like most of us have been outsmarted by her. The moment she stood on the balcony and hold the cup on which written "my house my rule", she declared her decisive victory and mocked the rest.
TLDR; Marta planed it since beginning, don't be fooled by her trick and lying POV. This film is like Life of Pi without final reveals of darkness, you need to be smart enough to figure it out by yourself.