Between the Lines
In the last episode, Tsukasa gets an extra line, not present in the manga, where he mentions that:
"I hope she does well, like Amane did".
Not with the exact same words, but the mention here is quite clear. During Tsukasa's speech, we see Hanako's knife highlighted.
If we consider the hypothetical situation, at first Shijima says that in order for them to leave that reality, it was necessary for Amane and Mitsuba to be killed.
"the nightmare/fairy tale ends when the protagonist dies, there is no story without him"
Seeing Tsukasa mention that Amane did a good job, as he wished Nene would do, we can imagine that the older brother may have felt obligated to kill Tsukasa.
As a real agreement about "the nightmare ending" for it to end, he needs to kill Tsukasa, the thing inside him, without fail.
It's interesting that Tsukasa mentioned this at that exact moment, as well as the knife being displayed as a clue lost between the words.
It doesn't mean that this is directly related to Mei or her ability (although her death is also conditional on the death or life of the Yugi twins, she was alive in the current reality), but it could be linked to some real situation between the twins.
What if this was like a preview?
Amane and Tsukasa would be trapped in that house and the only way for him and his family to escape would be to kill the boy, specifically the thing inside him. There would be no other condition besides that.
Just like Kou and Nene's dilemma, in order to return to reality, they would have to kill Amane and Mitsuba.
And as we saw, the rules could be broken, there was another option. Something similar to the context of the twins. Although Tsukasa did in fact die, he didn't disappear, he stuck to Hanako until Amane found another way to save his younger brother.
Leaving Tsukasa trapped (as well as Nene) in the boundary until he finds a solution.
Perhaps Tsukasa could be mentioning this, as Amane did his "job" well, killing him.
Or, it could just be a dramatization of a scene from the anime.
It's interesting to think about this, because it makes you wonder about the real situation of Tsukasa's death, if it was something Amane did because he wanted to save his brother from the entity, or if it was something Tsukasa expected Amane to do, to the point of perhaps causing his own death.
If Tsukasa hadn't found out that Amane would kill him, he wouldn't have insisted on going back to see how he would be killed, and upon returning, perhaps he purposefully did relevant actions that made Amane make the decision to kill him.
Don't take any of this seriously, it's just a brief analysis.