To start off with their mother, she's clearly why Mona developed the tendency to lie to appease. But their mother also lied, saying young Mona could see Naoi again once things settled down. I wonder if their mother caught on to Mona's compulsive lying and why she does it. It could be the real reason their mother invited Naoi to live with them, to hopefully stop Mona's lying before she's caught in one that causes serious trouble.
As for Kurumi and Naoi, neither gets the complete picture by themselves. Kurumi was too trusting, not even willing to believe she'd been lied to and attempted to be used to get Naoi to move in with Mona and their mother, until Mona outright confessed to that being the case. Meanwhile, due to their mother reaching out first, Naoi thinks that Mona is acting on her orders for some selfish reason, when in reality Mona is in a terrible spot and desperate to see her. Hopefully the words from the closing party of the festival about them figuring it out together works out.
As for Kokoro and Mona, Kokoro is being used as a mirror of where Mona could end up if her obsession with getting Naoi back into her life goes too far. It should probably be noted that the author did do an incest oneshot between half sisters in the past, so hopefully Mona viewing Naoi as the only one she has doesn't go in that direction. Kokoro did give Mona helpful advice, but Mona's former friend from school wouldn't accept her back after the lies, so now Mona's on a bad path. And hopefully, Kokoro doesn't backslide from getting involved with Mona. Unlike when Kudou approached her, Kokoro trusts Mona. And the fight this chapter showed that Kokoro hasn't dropped her animosity of Naoi. Even her saying she gave up on Kurumi was given an uncertain answer, the original text being " Yeah....I guess." And she's relying on Akane and Azuma, which might not keep working out. For example, I've seen Japanese fan art of Akane flirting with Azuma. So rather than get rejected by a friend like Mona did, she could end up feeling alone again if she starts to feel Iike the third wheel. Mona also doesn't realize the person Kokoro's former behavior was directed at was Kurumi, so she might keep accidentally creating situations that could lead to confrontations. Though hopefully, Kokoro just ends up as someone Mona can talk to.
The festival finally ended. Only one more chapter for the volume, but over 45 pages left, so either next chapter will be long, or volume 7 will have alot of bonus stories set during the festival. Based on the preview, break next month, so expect chapter 35 in November.