As weird as it might appear, people process things like this pretty regularly. Not only was the mom likely paying more "attention" to Rinne's sister, she was also made to be the secondary in her own thoughts of "my sister has it way worse, I should step aside, I shouldn't enjoy the things she can't, I don't need as much attention, etc". This is pretty common in these situations to some degree, but also tend to blow up eventually if not addressed. You can't just shut everything out of your life forever.