Okay so here me out i have 2 other theory than the abuse one :
In this chapter its clearly Tsukasa in the end but he didn't seem really bad or mean, to me he wasn't always like this maybe he had some issues as a child but not as bad as now. To me the twin's parents where rarely home, which can explain the fact that they seem to have a lot of toy's in their room (you can see it at the end of chapter 75) and it can also explain the fact that the crying kid which is probably young hanako care deeply about the toys, but what if Amane (young hanako) was a depressed child ? did you ever heard about depressed people trying to kill their family because to them the future is something horrible ? These people decide to kill them to make them avoid this terrible future. well maybe that what happen with Amane, he grow depressed without his parents and maybe he was tired and stressed to always look after his brother, we know that tsukasa was the type of child to fight eéasily or to run away, Amane himself told Yashiro that Tsukasa was violent everytime he try to prevent him from doing something, which is kinda normal for a child who grow without is parents around, so maybe hanako grow tired of all this and decide to take his brother with him so that they wouldn't have to live like this.
And my second theory take place with the same background but Hanako didn't kill tsukasa to avoid him from living this life maybe he kill him for another reason, in the flashback of Tsuchigomori's receptacle Hanako say that he will not be going anywhere and he was bruised, rope mark on his wrist and all, well maybe Hanako talk with tsukasa about leaving (cit can mean dying or maybe just leaving but alone). But Tsukasa didnt want Amane to leave which is also normal for a child who grow up alone with his brother, but we know that Tsukasa as anger issues and other issues, maybe he start getting violent to keep Amane here, he could have tie him up to make sure he stay by his side, Tsukasa could totally do something like that, maybe what some of us took for sexual harassement was just Tsukasa being obsessed by Hanako because he doesnt want to be left alone, which didn't justify his action let's be clear, he has issues, but this is also possible. And so Amane decided that if he wanted to leave he would have to take his brother with him because otherwise he wouldn't let him leave alone, the night after talking with tsuchigomori he decided to kill Tsukasa and to kill himself afterward.
After seeing their two child dead the parents would have also killed themself out of guilt from leaving them alone and sorrow from loosing their two child...
That's only a theory ! please dont be mean about it, i respect your theory so please respect mine too ! if y'all have other idea i would be happy to hear them !
Okay so here me out i have 2 other theory than the abuse one :
In this chapter its clearly Tsukasa in the end but he didn't seem really bad or mean, to me he wasn't always like this maybe he had some issues as a child but not as bad as now. To me the twin's parents where rarely home, which can explain the fact that they seem to have a lot of toy's in their room (you can see it at the end of chapter 75) and it can also explain the fact that the crying kid which is probably young hanako care deeply about the toys, but what if Amane (young hanako) was a depressed child ? did you ever heard about depressed people trying to kill their family because to them the future is something horrible ? These people decide to kill them to make them avoid this terrible future. well maybe that what happen with Amane, he grow depressed without his parents and maybe he was tired and stressed to always look after his brother, we know that tsukasa was the type of child to fight eéasily or to run away, Amane himself told Yashiro that Tsukasa was violent everytime he try to prevent him from doing something, which is kinda normal for a child who grow without is parents around, so maybe hanako grow tired of all this and decide to take his brother with him so that they wouldn't have to live like this.
And my second theory take place with the same background but Hanako didn't kill tsukasa to avoid him from living this life maybe he kill him for another reason,
in the flashback of Tsuchigomori's receptacle Hanako say that he will not be going anywhere and he was bruised, rope mark on his wrist and all, well maybe Hanako talk with tsukasa about leaving (cit can mean dying or maybe just leaving but alone). But Tsukasa didnt want Amane to leave which is also normal for a child who grow up alone with his brother, but we know that Tsukasa as anger issues and other issues, maybe he start getting violent to keep Amane here, he could have tie him up to make sure he stay by his side, Tsukasa could totally do something like that, maybe what some of us took for sexual harassement was just Tsukasa being obsessed by Hanako because he doesnt want to be left alone, which didn't justify his action let's be clear, he has issues, but this is also possible. And so Amane decided that if he wanted to leave he would have to take his brother with him because otherwise he wouldn't let him leave alone, the night after talking with tsuchigomori he decided to kill Tsukasa and to kill himself afterward.
After seeing their two child dead the parents would have also killed themself out of guilt from leaving them alone and sorrow from loosing their two child...
That's only a theory ! please dont be mean about it, i respect your theory so please respect mine too ! if y'all have other idea i would be happy to hear them !