As you all know, I think that Elly’s insistence that Mike devote most of his free time to watching over Lizzie is what makes him bitterly resent her. You also know that I think that Elly is far too stupid, self-absorbed and bedizened by the social message films that assure her that older siblings are only too happy to help Mommy out to realize that the only way that the Butches and Cindy Lous that peopled such reactionary pieces of trash lived in fear of the forced, repeated contact between their father’s belt and their bared glutei maximi and thus regarded the regrettable loss of their own childhoods as being a small price to pay to avoid getting abused by a drunken idiot patriarch who needed to feel like a big man.
Her inability to see that Mike might like to play with his friends unhindered by the unwelcome and enforced presence of someone who can’t participate in his games would be bad enough were it not for her insistence that he let her win every game they play so as to avoid having her cry. Since she’s only capable of looking at the surface of things, the image of an older brother giving way to someone who can’t keep up somehow appeals to her; she simply isn’t capable of ever realizing that by doing so, she’s feeding into the resentment that he’s had ever since he was told not to boast about all the great things that he can do so that everyone can coo over what looks to him like a helpless, wailing lump in pink who gets praised for doing nothing at all.
That being said, it isn’t just Mike’s not being able to even express his resentment without being treated like a monster that’s a problem; since Lizzie was used to being handed victories in the name of a false fairness, she’s quite unable to cope with a situation where she has to play by the same rules as everyone else. That is right, friends; I went there and I brought back souvenirs: Lizzie thinks that all of her teachers are finks because they don’t hand her A-pluses for showing up because by the time her personality gelled, she assumed that she was supposed to win every single competition she ever entered. I don’t want to take this too far by assuming that the mere fact that Thérèse actually wanted to keep her from doing what she wanted when she wanted to was what makes Liz think of her as being an irrational, cold-hearted monster but it might actually be that her need to demonize anyone in her way was formed because Elly wanted to avoid having to comfort her if Mike were to win a game they were playing.