What movie is this?

Okay so I'm having issues with figuring out what movie I'm thinking of

and I can't get it out of my head.

So it was like these two girls or whatever, and one day this lady

asked the other lady if she could pick up her son from school or

whatever and the second lady never came and got her son that night. I

think that the first lady gave the kid to his dad and the first lady

tries to go out there and find out where she went and I think that she

ended up killing her sister and drowning her in a lake or whatever.

She ends up finding her and she was arrested. And also the second lady

was leaving the first lady like notes or something to help find her or

whatever. And the reason why the second lady dissapeared was I think

because she got a letter or something at her work and it was like a

picture of her and i think her sister? It was released in 2017-2018

and I tried to look for it online and googling all of the movies that

were released through those times but I couldn't find it.

Please Help ME!!