Yeah. In the letter she tells her mother to have this planned conversation with brother (to fill him in on his part of the cover-up) somewhere away from home because she's worried about her mother's house and phone being bugged. But she didn't think "Hey, maybe I shouldn't leave this highly incriminating document in my jail cell." She is not a smart person, at all.

The wildest part about this case is that she really ought to have gotten away with it. The police absolutely botched the case. Kouri wasn't even charged until something like a year or two after the murder. The initial investigation failed to gather any meaningful evidence, basically just completely shit the bed on everything, overlooked a whole bunch of red flags, and she was free and clear until Eric's family hired a PI to look into it and get charges pressed.

The only reason she got convicted is that she's spectacularly stupid and left a trail of evidence a mile wide, so that even a year or so after the fact it will still possible to easily show that she did it.