Cinematic parallels (do Not @ me /hj):
Death Note (featuring Light Yagami & Sayu Yagami) and Frozen (featuring Elsa & Anna)
Sayu: "Why do you have to lock your door, Light?"
Anna: *watches with surprise and resignation as Elsa locks her door for the first time*
Doors getting locked for the first time when the elder sibling goes through something. (They were a literal open door to their younger sisters (despite being reserved, they happily allowed their younger sisters nearly full access to their heart))
I bet Sayu's "I need some help with my homework!" is equivalent to Anna's "Do you wanna build a snowman?"
Obviously Light & Elsa know it's an excuse to spend time together but how could they refuse their baby sister??
Light is a genius and socially intelligent enough to know Sayu is trying to 'manipulate' him into doing all her homework himself. Yet he allows that to happen anyway. Why?
Because he recognizes her primary motive: it's to spend time with him (how can he deny her that? When he wants the same??)
Meanwhile Elsa using her ice powers (before kid!Anna getting hit with her ice takes place) would still be frowned upon (they were at it at the crack of dawn, moreover this is their parents' reaction (they didn't know Anna's been hit at her forehead by Elsa yet, they merely saw the ice covered walls and Anna in Elsa's arms:
Elsa's father: "This is getting out of hand."
...which okay fair. But this means this has happened before (not Elsa hitting Anna accidentally of course, but Elsa playing with Anna using her ice powers (also evidenced by how Elsa is shushing Anna as they rush down the stairs to "build a snowman")) My point is Elsa knew their parents would not approve (even harmless playing around) yet she indulges Anna coz
A) Ice powers cool, not enough to defy parents which is why bigger reason:-
B) She ADORES Anna too much to think about rules)
Both Light & Elsa created distance after they realised that their seemingly harmless quality time of choice (ice powers could hit Anna - must avoid vs Sayu could accidentally touch the Death Note putting Light in a difficult position to contemplate killing her, must avoid as contrary to popular belief Light doesn't even wish to think on that) could actually hurt their respective younger sisters.
So even when Light does wish to spend time with Sayu, had no qualms about getting "manipulated" by her into doing all her homework BEFORE Ryuk dropped the bombshell which made him realize if he has to continue being Kira without risking killing his family- He HAS to distance himself from them so that they never find the fatal evidence- the Death Note.
This is why he rats Sayu out. And this is another similarity. The Yagami parents know this has happened before and do not approve obviously so there is less chance of Sayu getting close to his room for homework purposes thus eliminating her chance to encounter the killer notebook- which Light is counting on so that there is less probability of Sayu finding out the existence of Death Note and what that means for Kira's identity and hence! Light won't have to make the terrible choice of killing her.
It's fascinating how the distance between Elsa & Anna is created by their parents AND Elsa so that Elsa doesn't hurt anybody. Because Elsa's powers were known to her family apart from Anna after the memory wipe (wait now where have we heard losing memories of "magic" before? something something Yotsuba arc)
That's not true for Light though, he has to carry the burden of his choice of continuing to exercise the inherently evil power to kill all by himself (not justifying him at all- it's merely sad to see how much he wishes to use it to have the least negative impact/collateral damage like he ESPECIALLY wants to keep his family safe from himself...and yet we know as Ryuk points out that the Yagami family turns out to be the most tragic one, like you cannot "With Great Power comes Great Responsibility" out of this one Light! You have to leave that power alone! (No good way of using that power after all)
Still, the way Light comes up with incredibly complex plots (fake bottom drawer that just blows up in flames? yup) JUST SO he doesn't have to kill his family says a lot)
Idk just Light & Elsa both trying to protect their sisters from the harmful nature of their power in different ways but the underlying care is the same. It's the same!!! And it's making me insane.
(Want to write a meta (that nobody asked for) comparing Light & Elsa but just thought I should leave my half baked thoughts about sibling parallels here)