Trisha's little sister loves Trisha and she seems to know certain things which the mother herself does not know. Like the locker key's whereabouts. She may know much more about Trisha as she seems to hang with her and observe every little thing. From the mouths of babes.
That's what Kabir said as well and it's true. Often the older people will say things in front of young children that we think they don't understand and that's the biggest mistake. Just because their young, innocent, and naive doesn't mean that they won't understand what is happening or at least remember it.