I don't get it.
The writer has told us about heaps of things. There's so many unknown details, does the existence of unknown details as well as the existence of the mc's dead mom backstory have to all be linked?
Why is Jill's sister's story somehow inexplicably linked to the mc's dead mom? Because they are both dead? We now know they aren't one in the same person (a lot of us knew that from the start but there were plenty of stubborn people that would not accept it), so basically it's just unrelated information.
The backstories are relevant in their own context, but there's no obvious link to each other. It's definitely not "Chekhov's Gun". If that's how "Chekhov's Gun" was to be interpreted, it would mean every loose end must be all interrelated because the writer wrote it:
- We've been told about Maya's dad - maybe he's Lynette's brother...
- We randomly met Nora in a pub - maybe she's Lynette's sister...
- We've been given vague information about Sage's parents - maybe Lynette's not dead and she's Sage's mom...
- We've been told that Bella inherited her home from wealthy parents - maybe she's actually Lynette after faking her death and having a face lift...
- We haven't blatantly seen Quinn's mom (I believe we have but it's debateable) - maybe Lynette was living a double life and she's Quinn's mom too...
- Who's sending incriminating photos to the Jocks? - Maybe it's Lynette's ghost...
- What's behind Bella's locked door? - Maybe it's something to do with Lynette...
- Who was skulking outside the MC's room in episode 6? - Maybe Lynette isn't dead and she's stalking the mc...
- Who trashed the DIKs mansion? - Maybe it was Lynette trying to get the mc's attention...
- How did Lana die? - Maybe she and Lynette were friends Lana committed suicide when Lynette died giving birth to the mc...
- Who is the Madame? - Maybe it's Lynette's mom fallen on hard times (since the Madame looks like an old hag)...
- Jill's sister Lana was mentioned, the mc's mom Lynette was mentioned - maybe they are intertwined somehow...
All of the above must be true otherwise why did the writer write about them? No. Everything written in the story does not have to be interrelated. The backstory of one character is used to define that character. When we learn about Jill's dead sister we learn more about Jill.
"Why the writer tell us about the sister of Jill, and the mother of the MC and her diary, suggesting that could be some kind of link?"
The absolute misconception here is that DPC ever suggested that there could be some kind of link.
It's only people on this thread that continuously believe there was a link and propagate that theory until people think it is real. There's nothing in the story that suggests there's a link at all.
First some people were 100% convinced that Jill's dead sister was the mc's mom. Why? Because they both died young and they both came from wealthy families. Too bad people kept providing plenty of
evidence to the contrary, but those "Aunt Jill" theorists would not let up.
Now we find out for a fact that they are two separate people, and instead of saying, "Fuck, we were definitely barking up the wrong tree with that theory", these crazy doomsday theorists just change the date of Judgement Day, rather than realise they are totally fucking wrong.
It's not impossible that Lynette was somehow known to Bella or Jill's sister, just like it's not impossible for a lot of other substantiated, currently not even remotely alluded to outcomes could be true, but it's not Chekhov's Gun, not even remotely. If you listen carefully you can hear Anton Chekhov turning in his grave right now at the blatant misuse of his theory.