Lexy was shot and things were wrapping up with the husband being taken away yelling, I turned to my wife and said "I bet Anna is actually the killer and she was framing Catherine/Lexy". I was kind of close, I guess...

I didn't hate the twist, but agree it felt kind of cheap. But I did like that they actually worked in her years of being overlooked as a housekeeper, and her faking her dementia. Made it feel a little less cheap..

blackmailing Catherine/Lexy over the circumstances surrounding her sister's death.

The whole plot was completely unbelievable and was constructed in a way, only to keep the audience guessing, although is was nonsensical at times. The reveal of Lexy was just stupid. She and Anna are kind of co-workers, but Anna never says to herself well she looks and sounds like the girl I used to go to school with for years, but she is skinny now, but her mother recognized her from TV because she was her cleaning lady. The actress Rebecca Rittenhouse is obviously younger than Tessa Thompson. Marin Ireland and Poppy Liu. To make the audience believe she's a killer they throw in the flashback of her "killing" her sister, Lexy knows she's not the killer, so why invite Anna to her cabin in the first place, since she is most likely the killer in Lexy's eyes?
Even the place of the showdown was so stupid. Anna gets thrown out of her hotel, but she's not staying with her mother, she drives to a remote cabin.
Anna married the brother of the woman who had her raped or was part of the group at least? Anna has to believe that Jack is the killer, since she saw them together at the crime scene. The morning after she decides to go back to work, not knowing that Rachel was murdered and she would go back to her home town.
Jack knows that he is not the killer, but he barely investigates anyone else. He tells Boston to investigate the husband, who is the logical suspect, not only in Jack's mind, but in the mind of any investigator, except Jack's partner who kind of investigates no one, until she stumbles on Jack's weird behavior. And now one of the most stupid things. Jack swaps his nieces DNA, so he doesn't get matched as the guy who had sex with Rachel. That makes the investigation much easier, because the crime lab would tell the police that the killer is a close relative of the lead detective.
There are probably a thousand details which make the show stupid, but the final twist is the worst. Yes, it's kind or cool that it was a perfect crime by Anna's mother, to make her daughter happy again, but it didn't make sense, since she could not know that Anna had returned to work, so Rachel's murder would not have brought back Anna. Rachel had no defensive wounds, although she fought an elderly woman and the knife only killed her because of the many wounds.